Proton conducting membrane, process for its production, and fuel cells made by using the same

ABSTRACT

The present invention provides relates to a crosslinkable, proton-conducting membrane having a crosslinked structure, excellent in heat resistance, durability, dimensional stability and fuel barrier characteristics, and showing excellent proton conductivity at high temperature, characterized by comprising (a) an organic/inorganic hybrid structure (A) covalently bonded to 2 or more silicon-oxygen crosslinks and, at the same time, having a carbon atom, and (b) an acid containing structure (B) having an acid group, covalently bonded to a silicon-oxygen crosslink and having an acidic group; and provides a fuel cell using the same membrane.  
     The present invention also provides a method for producing the proton-conducting membrane, comprising steps of preparing a mixture containing an organic/inorganic hybrid, crosslinkable compound (C) and compound (D), the former having 2 or more crosslinkable silyl groups and carbon atoms each being bonded to the silyl group via the covalent bond and the latter having a crosslinkable silyl group and acid group, as the first step; forming the above mixture into a film as the second step; and hydrolyzing/condensing or only condensing the hydrolyzable silyl group contained in the mixture formed into the film to form a crosslinked structure as the third step.

FIELD OF THE INVENTION

[0001] This invention relates to a proton (hydrogen ion)-conductingmembrane, method for producing the same, and fuel cell using the same,more particularly the proton-conducting membrane, excellent in heatresistance, durability, dimensional stability and fuel barriercharacteristics, and showing excellent proton conductivity at hightemperature, method for producing the same, and fuel cell using thesame, and, at the same time, the proton-conducting membrane for thedirect fuel type fuel cell which is directly supplied with fuel, e.g.,methanol or methane, method for producing the same, and fuel cell usingthe same.

BACKGROUND OF THE INVENTION

[0002] Recently, the fuel cell has been attracting attention as a powergenerating device of the next generation, which can contribute tosolution of the problems related to environments and energy, now havingbeen increasingly becoming serious social problems, because of its highpower generation efficiency and compatibility with the environments.

[0003] Fuel cells generally fall into several categories by electrolytetype. Of these, a polymer electrolyte fuel cell (sometimes referred toas PEFC), being more compact and generating higher output than any othertype, is considered to be a leading fuel cell type in the future forvarious purposes, e.g., small-size on-site facilities, and as powersources for movable applications (e.g., vehicles) and portableapplications.

[0004] Thus, PEFCs have inherent advantages in principle, and are beingextensively developed for commercialization. PEFCs normally use hydrogenas the fuel. Hydrogen is dissociated into proton (hydrogen ion) andelectron in the presence of catalyst provided on the anode side. Ofthese, the electron is passed to the outside, where it is used aselectricity, and circulated back to the system on PEFC's cathode side.On the other hand, the proton is passed to the proton conductingmembrane (electrolyte membrane), through which it moves towards thecathode side. On the cathode side, the proton, electron recycled backfrom the outside and oxygen supplied from the outside are bonded to eachother in the presence of catalyst, to produce water. Thus, a PEFC byitself is a very clean energy source which generates power while it isproducing water from hydrogen and oxygen.

[0005] Hydrogen to be supplied to a fuel cell is normally produced by anadequate method, e.g., methanol reforming to extract hydrogen. However,the direct fuel type fuel cell has been also extensively developed. Itis directly supplied with methanol or the like, from which the protonand electron are produced in the presence of catalyst, where water isnormally used together with methanol.

[0006] In the fuel cell, the proton conducting membrane is responsiblefor transferring the proton produced on the anode to the cathode side.As described above, flow of the proton takes place in concert with thatof the electron. It is therefore necessary to conduct a sufficientquantity of the proton at high speed, for the PEFC to produce highoutput (or high current density). Therefore, it is not too much to saythat performance of the proton conducting membrane is a key toperformance of the PEFC. The proton conducting membrane also works asthe insulation film which electrically insulates the anode and cathodefrom each other and as the fuel barrier membrane which prevents the fuelto be supplied to the anode side from leaking to the cathode side, inaddition to transferring the proton.

[0007] The proton conducting membranes for the current PEFCs are mainlyof fluorine resin-based ones, with a perfluoroalkylene as the mainskeleton, and partly with sulfonic acid group at the terminal of theperfluorovinyl ether side chains. Several types of these sulfonatedfluorine resin-based membranes have been proposed, e.g., Nafion^(R)membrane (Du Pont, U.S. Pat. No. 4,330,654), Dow membrane (Dow Chemical,Japanese Patent Application Laid-Open No.4-366137), Aciplex^(R) membrane(Asahi Chemical Industries, Japanese Patent Application Laid-OpenNo.6-342665), and Flemion^(R) membrane (Asahi Glass).

[0008] These fluorine resin-based membranes are considered to have aglass transition temperature (Tg) of around 130° C. under a humidifiedcondition, under which they work. The so-called creep phenomenon occursas temperature increases from the above level to cause problems, e.g.,changed proton conducting structure in the membrane to prevent themembrane from stably exhibiting the proton conducting performance, andmodification of the membrane to a swollen morphology, or jelly-likemorphology to make it very fragile and possibly cause failure of thefuel cell.

[0009] For these reasons, the maximum allowable temperature for stableoperation for extended periods is normally considered to be 80° C.

[0010] A fuel cell, depending on the chemical reaction for its workingprinciple, has a higher energy efficiency when it operates at highertemperature. In other words, a fuel cell operating at higher temperaturebecomes more compact and lighter for the same output. Moreover, a fuelcell operating at high temperature allows utilization of its waste heatfor cogeneration to produce power and heat, thus drastically enhancingits total energy efficiency. It is therefore considered that operatingtemperature of a fuel cell is desirably increased to a certain level,normally to 100° C. or higher, in particular 120° C. or higher.

[0011] The catalyst in service on the anode side may be deactivated byimpurities in the hydrogen fuel (e.g., carbon monoxide), a phenomenonknown as catalyst poisoning, when it is not sufficiently purified. Thisis a serious problem which can determine lifetime of the PEFC itself. Itis known that the catalyst poisoning can be avoided when the fuel celloperates at sufficiently high temperature, and the cell is preferablyoperated at high temperature also from this point of view. Moreover, theactive metals for the catalyst itself will not be limited to pure noblemetals, e.g., platinum, but can be extended to alloys of various metals,when the fuel cell can operate at sufficiently high temperature.Therefore, operability at high temperature is advantageous also viewedfrom reducing cost and widening applicable resources.

[0012] For the direct fuel type fuel cell, various approaches to extractthe proton and electron from the fuel directly and efficiently have beenstudied. It is a consensus that production of sufficient power isdifficult at low temperature, and possible when temperature is increasedto, e.g., 150° C. or higher.

[0013] Thus, operability of PEFCs at high temperature is demanded fromvarious aspects. Nevertheless, however, its operating temperature islimited to 80° C. by the heat resistance consideration of the protonconducting membrane, as discussed above at present.

[0014] The reaction taking place in a fuel cell is exothermic in nature,by which is meant that temperature within the cell spontaneouslyincreases as the cell starts to work. However, the PEFC must be cooledso as not to be exposed to high temperature of 80° C. or higher, aslimited by the resistance of the proton conducting membrane to heat. Itis normally cooled by a water-cooling system, and the PEFC's bipolarplate is devised to include such a system. This tends to increase sizeand weight of the PEFC as a whole, preventing it to fully exhibit itsinherent characteristics of compactness and lightness. In particular, itis difficult for a water-cooling system as the simplest cooling means toeffectively cool the cell, when its maximum allowable operatingtemperature is set at 80° C. If it is operable at 100° C. or higher, itshould be effectively cooled by use of heat of vaporization of water,and water could be recycled for cooling to drastically reduce itsquantity, leading to reduced size and weight of the cell. When a PEFC isused as the energy source for a vehicle, the radiator size and coolingwater volume could be greatly reduced when the cell is controlled at100° C. or higher, compared to when it is controlled at 80° C.Therefore, the PEFC operable at 100° C. or higher, i.e., the protonconducting membrane having a heat resistance of 100° C. or higher, isstrongly in demand.

[0015] As described above, the PEFC operable at higher temperature,i.e., increased heat resistance of the proton conducting membrane, isstrongly in demand viewed from various aspects, e.g., power generationefficiency, cogeneration efficiency, cost, resources and coolingefficiency. Nevertheless, however, the proton conducting membrane havinga sufficient proton conductivity and resistance to heat has not beendeveloped so far.

[0016] With these circumstances as the background, a variety ofheat-resistant proton conducting membrane materials have been studiedand proposed to increase operating temperature of PEFCs.

[0017] Some of more representative ones are heat-resistantaromatic-based polymers to replace the conventional fluorine-basedmembranes. These include polybenzimidazole (Japanese Patent ApplicationLaid-Open No.9110982), polyether sulfone (Japanese Patent ApplicationLaid-Open Nos.1021943 and 10-45913), and polyetheretherketone (JapanesePatent Application Laid-Open No.9-87510).

[0018] These aromatic-based polymers have an advantage of limitedstructural changes at high temperature. However, many of them have thearomatic structure directly incorporated with sulfonic acid orcarboxylic acid group. They tend to suffer notable desulfonation ordecarboxylation at high temperature, and are unsuitable for themembranes working at high temperature.

[0019] Moreover, many of these aromatic-based polymers have noion-channel structure, as is the case with fluorine resin-basedmembranes. As a result, it is necessary to incorporate a large number ofacid groups in these polymers, for them to sufficiently exhibit protonconductivity, causing problems, e.g., deterioration of membranestability and stability to hot water, and, in some cases, dissolution ofthese polymers in hot water. Moreover, the membranes of these polymerstend to be notably swollen as a whole in the presence of water, causingvarious problems, e.g., high possibility of separation of the membranefrom the electrode joint and broken membrane due to the stress producedat the joint in the membrane-electrode assembly, resulting from the dryand wet conditional cycles which change the membrane size, andpossibility of deteriorated strength of the water-swollen membrane,leading to its failure. In addition, each of the aromatic polymers isvery rigid in a dry condition, possibly causing damages and otherproblems while the membrane-electrode assembly is formed.

[0020] On the other hand, the following inorganic materials have beenalso proposed as the proton conducting materials. For example, Minami etal. incorporate a variety of acids in hydrolyzable silyl compounds toprepare inorganic proton conducting materials (Solid State Ionics, 74(1994), pp.105). They stably show proton conductivity even at hightemperature, but involve several problems; e.g., they tend to be crackedwhen made into a thin film, and difficult to handle and make them into amembrane-electrode assembly.

[0021] Several methods have been proposed to overcome these problems.For example, the proton conducting inorganic material is crushed to bemixed with an elastomer (Japanese Patent Application Laid-OpenNo.8-249923) or with a polymer containing sulfonic acid group (JapanesePatent Application Laid-Open No. 10-69817). However, these methods havetheir own problems. For example, the polymer as the binder for each ofthese methods is merely mixed with an inorganic crosslinked compound,and has basic thermal properties not much different from those of thepolymer itself, with the result that it undergoes structural changes ina high temperature range, failing to stably exhibit proton conductivity,and its proton conductivity is generally not high.

[0022] A number of R & D efforts have been made for various electrolytemembranes to solve these problems involved in the conventional PEFCs.None of them, however, have succeeded in developing proton conductingmembranes showing sufficient durability at high temperature (e.g., 100°C. or higher) and satisfying the mechanical and other properties.

[0023] In the direct methanol type fuel cell (sometimes referred to asDMFC) which works on methanol as the fuel in place of hydrogen, on theother hand, methanol directly comes into contact with the membrane. Thesulfonated fluorine resin-based membrane, e.g., Nafion^(R) membrane, nowbeing used has a strong affinity for methanol, possibly causing problemswhich can lead to failure of the fuel cell when it absorbs methanol,e.g., swelling to a great extent and dissolution in methanol in somecases. Crossover of methanol to the oxygen electrode side can greatlyreduce cell output. These problems are common also with the electrolytemembranes containing an aromatic ring. Therefore, the membranesdeveloped so far are neither efficient nor durable also for DMFCs.

[0024] It is an object of the present invention to provide a protonconducting membrane, excellent in heat resistance, durability,dimensional stability and fuel barrier characteristics, and showingexcellent proton conductivity at high temperature. It is another objectof the present invention to provide a method for producing the same. Itis still another object of the present invention to provide a fuel cellusing the same.

DISCLOSURE OF THE INVENTION

[0025] The inventors of the present invention have found, after havingextensively studied a variety of electrolyte membrane to solve the aboveproblems, that an innovative organic/inorganic hybrid membraneunprecedentedly excellent in heat resistance, durability, dimensionalstability and fuel barrier characteristics, and showing excellent protonconductivity even at high temperature can be obtained by including, asthe essential components for the proton conducting membrane, acrosslinked structure of specific organic/norganic hybrid structure andacid-containing crosslinked structure. The present invention has beendeveloped based on the above knowledge.

[0026] The first aspect of the present invention is a proton conductingmembrane crosslinkable and having a crosslinked structure by thesilicon-oxygen bond, wherein the proton conducting membrane comprises

[0027] (a) an organic/inorganic hybrid structure (A) covalently bondedto 2 or more silicon-oxygen crosslinks and having a carbon atom, and

[0028] (b) an acid containing structure (B) having an acid group,covalently bonded to a silicon-oxygen crosslink and having an acidgroup.

[0029] The second aspect of the present invention is the protonconducting membrane of the first aspect, wherein the organic/inorganichybrid structure (A) is represented by the general formula (1):

[0030] (wherein, X is an —O— bond or OH group involved in thecrosslinking; R¹ is a carbon-containing group of 1 to 50 carbon atoms;R² is methyl, ethyl, propyl or phenyl group; and “n” is an integer of 0,1 or 2).

[0031] The third aspect of the present invention is the protonconducting membrane of the second aspect, wherein R¹ in the generalformula (1) is a hydrocarbon group.

[0032] The fourth aspect of the present invention is the protonconducting membrane of the third aspect, wherein R¹ in the generalformula (1) has a structure represented by the general formula (3):

[0033] (wherein, “n” is an integer of 1 to 30).

[0034] The fifth aspect of the present invention is the protonconducting membrane of the fourth aspect, wherein the organic/inorganichybrid structure (A) is represented by the general formula (4):

[0035] (wherein, X is an —O— bond or OH group involved in thecrosslinking; and “n” is an integer of 0, 1 or 2).

[0036] The sixth aspect of the present invention is the protonconducting membrane of the second aspect, wherein R¹ in the generalformula (1) has a siloxane structure.

[0037] The seventh aspect of the present invention is the protonconducting membrane of the sixth aspect, wherein R¹ in the generalformula (1) is represented by the general formula (5):

[0038] (wherein, R⁵ and R⁶ are each methyl, ethyl, propyl or phenylgroup, which may be the same or different; and “n” is an integer of 1 to20).

[0039] The eighth aspect of the present invention is the protonconducting membrane of the first aspect, wherein the structure (B)containing an acid group is represented by the general formula (2):

[0040] (wherein, X is an —O— bond or OH group involved in thecrosslinking; R³ is a molecular chain group having at least one acidgroup; R⁴ is methyl, ethyl, propyl or phenyl group; and “m” is aninteger of 0, 1 or 2).

[0041] The ninth aspect of the present invention is the protonconducting membrane of the eighth aspect, wherein the acid group whichR³ in the general formula (2) has is sulfonic acid group.

[0042] The tenth aspect of the present invention is the protonconducting membrane of the ninth aspect, wherein R³ in the generalformula (2) is represented by the general formula (6):

[0043] (wherein, “n” is an integer of 1 to 20).

[0044] The 11^(th) aspect of the present invention is the protonconducting membrane of the tenth aspect, wherein “n” in the generalformula (6) is 3.

[0045] The 12^(th) aspect of the present invention is the protonconducting membrane of one of the first to 11^(th) aspects which isfurther composited with a fibrous material (I).

[0046] The 13^(th) aspect of the present invention is the protonconducting membrane of the 12^(th) aspect, wherein the fibrous material(I) is composed of a short fibrous material (J) anchor long fibrousmaterial (K).

[0047] The 14^(th) aspect of the present invention is the protonconducting membrane of the 12^(th) aspect, wherein the fibrous material(I) is surface-treated with a silane coupling agent to have aproton-conductive surface.

[0048] The 15^(th) aspect of the present invention is the protonconducting membrane of the 12^(th) aspect, wherein the fibrous material(I) is composed of glass fibers.

[0049] The 16^(th) aspect of the present invention is the protonconducting membrane of the 15^(th) aspect, wherein the glass fibers areof alkali- or acid-resistant glass.

[0050] The 17^(th) aspect of the present invention is the protonconducting membrane of the 13^(th) aspect, wherein the long fibrousmaterial (K) is composed of glass fibers in the form of woven fabric,non-woven fabric or glass fiber paper produced by a paper-makingprocess.

[0051] The 18^(th) aspect of the present invention is the protonconducting membrane of the 17^(th) aspect, wherein the long fibrousmaterial (K) is in the form of thirled, square-weave fabric.

[0052] The 19^(th) aspect of the present invention is the protonconducting membrane of the 17^(th) aspect, wherein the long fibrousmaterial (K) has a thickness of 300 μm or less.

[0053] The 20^(th) aspect of the present invention is the protonconducting membrane of the 12^(th) aspect, wherein the short fibrousmaterial (J) is incorporated at 1 to 75% by weight on theorganic/inorganic hybrid structure (A) and acid-containing structure (B)totaled.

[0054] The 21^(st) aspect of the present invention is the protonconducting membrane of the 13^(th) aspect, wherein the short fibrousmaterial (J) is composed of whiskers (L) and/or short glass fibers (M).

[0055] The 22^(nd) aspect of the present invention is the protonconducting membrane of the 21^(st) aspect, wherein the whiskers (L) havea diameter of 0.1 to 3 μm, length of 1 to 20 cm and aspect ratio of 5 to100.

[0056] The 23^(rd) aspect of the present invention is the protonconducting membrane of the 21^(st) or 22^(nd) aspect, wherein thewhiskers (L) are of boron carbide, silicon carbide, alumina, aluminumborate, silicon nitride or K₂O.6TiO₂.

[0057] The 24^(th) aspect of the present invention is a method forproducing the proton conducting membrane of one of the first to 23^(rd)aspect, comprising steps of preparing a mixture containing anorganic/inorganic hybrid, crosslinkable compound (C) and compound (D),the former having 2 or more crosslinkable silyl groups and carbon atomseach being bonded to the silyl group via the covalent bond and thelatter having a crosslinkable silyl group and acid group, as the firststep; forming the above mixture into a film as the second step; andhydrolyzing/condensing or only condensing the hydrolyzable silyl groupcontained in the mixture formed into the film to form a crosslinkedstructure as the third step.

[0058] The 25^(th) aspect of the present invention is a method forproducing the proton conducting membrane of one of the first to 23^(rd)aspect, comprising steps of preparing a mixture containing anorganic/inorganic hybrid, crosslinkable compound (C) and compound (E),the former having 2 or more crosslinkable silyl groups and carbon atomseach being bonded to the silyl group via the covalent bond and thelatter having a crosslinkable silyl group and mercapto group, as thefirst step; forming the above mixture into a film as the second step;hydrolyzing and condensing the hydrolyzable silyl group contained in themixture formed into the film to form a crosslinked structure as thethird step; and oxidation of the mercapto group in the crosslinkedstructure obtained in the third step into sulfonic acid as the fourthstep.

[0059] The 26^(th) aspect of the present invention is a method forproducing the proton conducting membrane of one of the first to 23^(rd)aspect, comprising steps of preparing a mixture containing anorganic/inorganic hybrid, crosslinkable compound (C) and compound (F),the former having 2 or more crosslinkable silyl groups and carbon atomseach being bonded to the silyl group via the covalent bond and thelatter having a crosslinkable silyl group and polysulfide group, as thefirst step; forming the above mixture into a film as the second step;hydrolyzing and condensing the hydrolyzable silyl group contained in themixture formed into the film to form a crosslinked structure as thethird step; and oxidation of the polysulfide group in the crosslinkedstructure obtained in the third step into sulfonic acid as the fourthstep.

[0060] The 27^(th) aspect of the present invention is a method forproducing the proton conducting membrane of one of the first to 23^(rd)aspect, comprising steps of preparing a mixture containing anorganic/inorganic hybrid, crosslinkable compound (C) and compound (H),the former having 2 or more crosslinkable silyl groups and carbon atomseach being bonded to the silyl group via the covalent bond and thelatter having a crosslinkable silyl group and halogen group, as thefirst step; forming the above mixture into a film as the second step;hydrolyzing and condensing the hydrolyzable silyl group contained in themixture formed into the film to form a crosslinked structure as thethird step; and substitution of the halogen group in the crosslinkedstructure obtained in the third step with sulfonic acid group as thefourth step.

[0061] The 28^(th) aspect of the present invention is the method of oneof the 24^(th) to 27^(th) aspects for producing the proton conductingmembrane, wherein the organic/inorganic hybrid, crosslinkable compound(C) is represented by the general formula (7):

[0062] (wherein, R¹ is a carbon-containing group of 1 to 50 carbonatoms; R² is methyl, ethyl, propyl or phenyl group; R⁵ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “n” is 0, 1 or 2).

[0063] The 29^(th) aspect of the present invention is the method of the28^(th) aspect for producing the proton conducting membrane, wherein R¹in the general formula (7) is a hydrocarbon group.

[0064] The 30^(th) aspect of the present invention is the method of the29^(th) aspect for producing the proton conducting membrane, wherein theorganic/inorganic hybrid, crosslinkable compound (C) is represented bythe general formula (8):

[0065] (wherein, R² is methyl, ethyl, propyl or phenyl group; R⁵ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is an integer of 1 to 30;and “n” is 0, 1 or 2).

[0066] The 31^(st) aspect of the present invention is the method of the30^(th) aspect for producing the proton conducting membrane, wherein theorganic/inorganic hybrid, crosslinkable compound (C) is represented bythe general formula (9):

[0067] (wherein, R⁵ is OCH₃ or OC₂H₅ group; and “n” is 0, 1 or 2).

[0068] The 32^(nd) aspect of the present invention is the method of the28^(th) aspect for producing the proton conducting membrane, wherein theorganic/inorganic hybrid, crosslinkable compound (C) is represented bythe general formula (10):

[0069] (wherein, R¹¹, R¹², R¹³ and R¹⁴ are each methyl, ethyl, propyl orphenyl group, which may be the same or different; R⁵ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is an integer of 1 to 30; and “n”is 0, 1 or 2).

[0070] The 33^(rd) aspect of the present invention is the method of the32^(nd) aspect for producing the proton conducting membrane, wherein theorganic/inorganic hybrid, crosslinkable compound (C) is represented bythe general formula (11):

[0071] (wherein, R⁵ is Cl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group;“m” is an integer of 1 to 30; and “n” is 0, 1 or 2).

[0072] The 34^(th) aspect of the present invention is the method of the24^(th) aspect for producing the proton conducting membrane, wherein theacid containing compound (D) is represented by the general formula (12):

[0073] (wherein, R³ is a molecular chain group having at least one acidgroup; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is 0, 1 or 2).

[0074] The 35^(th) aspect of the present invention is the method of the34^(th) aspect for producing the proton conducting membrane, wherein theacid containing compound (D) is sulfonic acid group.

[0075] The 36^(th) aspect of the present invention is the method of the35^(th) aspect for producing the proton conducting membrane, wherein theacid containing compound (D) is represented by the general formula (13):

[0076] (wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; X is hydrogen, an alkalimetal, alkyl group or ammonium group; “m” is 0, 1 or 2; and “n” is aninteger of 1 to 20).

[0077] The 37^(th) aspect of the present invention is the method of the36^(th) aspect for producing the proton conducting membrane, wherein “n”in the general formula (13) is 3.

[0078] The 38^(th) aspect of the present invention is the method of the24^(th) aspect for producing the proton conducting membrane, wherein theorganic/inorganic hybrid, crosslinkable compound (C) and acid containingcompound (D) are incorporated in a mixing ratio of 9:1 to 1:9 by weight.

[0079] The 39^(th) aspect of the present invention is the method of the25^(th) aspect for producing the proton conducting membrane, wherein thecompound (E) having mercapto group is represented by the general formula(14):

[0080] (wherein, R⁷ is a molecular chain group having at least onemercapto group; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0081] The 40^(th) aspect of the present invention is the method of the39^(th) aspect for producing the proton conducting membrane, wherein thecompound (E) having mercapto group is represented by the general formula(15):

[0082] (wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is 0, 1 or 2; and “n” isan integer of 1 to 20).

[0083] The 41^(st) aspect of the present invention is the method of the25^(th) aspect for producing the proton conducting membrane, wherein thecompound (E) having mercapto group is represented by the general formula(16):

[0084] (wherein, R⁶ is H, or CH₃, C₂H₅, C₃H₇ or C₆H₅ group; R⁴ is CH₃,C₂H₅, C₃H₇, C₆H₅, OH, OCH₃, OC₂H₅, OC₆H₅ group, or O—Si bond; “m” is aninteger of 1 to 20; and “n” is an integer of 3 to 500).

[0085] The 42^(nd) aspect of the present invention is the method of the41^(st) aspect for producing the proton conducting membrane, wherein R⁴,“m” and “n” in the general formula (16) are OCH₃ group, 3 and an integerof 3 to 100, respectively.

[0086] The 43^(rd) aspect of the present invention is the method of the41^(st) aspect for producing the proton conducting membrane, wherein R⁴,“m” and “n” in the general formula (16) are CH₃ group, 3 and an integerof 3 to 300, respectively.

[0087] The 44^(th) aspect of the present invention is the method of the25^(th) aspect for producing the proton conducting membrane, wherein thecompound (E) having mercapto group is represented by the general formula(17):

[0088] (wherein, R⁶ is H, or CH₃, C₂H₅, C₃H₇ or C₆H₅ group; R⁴ is CH₃,C₂H₅, C₃H₇, C₆H₅, OH, OCH₃, OC₂H₅ or OC₆H₅ group, R¹¹ is a substitute of6 carbon atoms or less; “m” is an integer of 1 to 20; “n” is an integerof 3 to 500; and “n+x” is an integer of 500 or less, where the unitcontaining mercapto group and that containing R¹¹ may be present in ablock or random form).

[0089] The 45^(th) aspect of the present invention is the method of the44^(th) aspect for producing the proton conducting membrane, wherein R⁴,“m” and “n+x” in the general formula (17) are OCH₃ group, 3 and aninteger of 50 or less, respectively.

[0090] The 46^(th) aspect of the present invention is the method of the25^(th) aspect for producing the proton conducting membrane, wherein theorganic/inorganic hybrid, crosslinkable compound (C) and compound (E)having mercapto group are incorporated in a mixing ratio of 9:1 to 1:9by weight.

[0091] The 47^(th) aspect of the present invention is the method of the26^(th) aspect for producing the proton conducting membrane, wherein thecompound (F) having a polysulfide group is represented by the generalformula (18):

[0092] (wherein, R⁸ is a molecular chain group having at least onepolysulfide group; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ isCl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0093] The 48^(th) aspect of the present invention is the method of the47^(th) aspect for producing the proton conducting membrane, wherein thecompound (F) having a polysulfide group is represented by the generalformula (19):

[0094] (wherein, R⁹ is a molecular chain group having at least onepolysulfide group; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ isCl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0095] The 49^(th) aspect of the present invention is the method of the48^(th) aspect for producing the proton conducting membrane, wherein thecompound (F) having a polysulfide group is represented by the generalformula (20):

[0096] (wherein, R¹⁰ is a polysulfide group; R⁴ is methyl, ethyl, propylor phenyl group; R⁶ is Cl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group;“m” is 0, 1 or 2; and “n” is an integer of 1 to 6).

[0097] The 50^(th) aspect of the present invention is the method of the49^(th) aspect for producing the proton conducting membrane, wherein “n”in the general formula (20) is 3.

[0098] The 51^(st) aspect of the present invention is the method of oneof the 47^(th) to 50^(th) aspects for producing the proton conductingmembrane, wherein the polysulfide group is tetrasulfide group(—S—S—S—S—).

[0099] The 52^(nd) aspect-of the present invention is the method of oneof the 47th to 50^(th) aspects for producing the proton conductingmembrane, wherein the polysulfide group is disulfide group (—S—S—).

[0100] The 53^(rd) aspect of the present invention is the method of the26^(th) aspect for producing the proton conducting membrane, wherein theorganic/inorganic hybrid, crosslinkable compound (C) and compound (F)having a polysulfide group are incorporated in a mixing ratio of 95:5 to10:90 by weight.

[0101] The 54^(th) aspect of the present invention is the method of oneof the 24^(th) to 27^(th) aspects for producing the proton conductingmembrane, wherein a crosslinking agent (G) of hydrolyzable, metalliccompound is used for the first step.

[0102] The 55^(th) aspect of the present invention is the method of the54^(th) aspect for producing the proton conducting membrane, wherein thecrosslinking agent (G) is of a compound represented by the generalformula (21):

[0103] (wherein, R⁶ is CH₃ or C₂H₅ group; and “m” is an integer of 1 to300).

[0104] The 56^(th) aspect of the present invention is the method of the54^(th) aspect for producing the proton conducting membrane, wherein thecrosslinking agent (G) is of a hydrolyzable, metallic compound havingTi, Zr or Al.

[0105] The 57^(th) aspect of the present invention is the method of the27^(th) aspect for producing the proton conducting membrane, wherein thecompound (H) having a halogen group is represented by the generalformula (22):

[0106] (wherein, R¹² is a molecular chain group having at least onehalogen group; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, orOCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0107] The 58^(th) aspect of the present invention is the method of the57^(th) aspect for producing the proton conducting membrane, wherein thecompound (H) having a halogen group is represented by the generalformula (23):

[0108] (wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; X is Cl, Br or I; “n” is aninteger of 1 to 20; and “m” is 0, 1 or 2).

[0109] The 59^(th) aspect of the present invention is the method of the27^(th) aspect for producing the proton conducting membrane, wherein theorganic/inorganic hybrid, crosslinkable compound (C) and compound (H)having a halogen group are incorporated in a mixing ratio of 9:1 to 1:9by weight.

[0110] The 60^(th) aspect of the present invention is the method of oneof the 24^(th) to 27^(th) aspects for producing the proton conductingmembrane, wherein a step for aging at 100 to 300° C. is included as apost-treatment step.

[0111] The 61^(st) aspect of the present invention is the method of oneof the 24^(th) to 27^(th) aspects for producing the proton conductingmembrane, wherein the short fibrous material (J) is incorporated in themixture in the first step, when it is incorporated as the fibrousmaterial (I) to be composited with the proton-conducting membrane.

[0112] The 62^(nd) aspect of the present invention is the method of oneof the 49th to 60^(th) aspects for producing the proton conductingmembrane, wherein the long fibrous material (K) is loaded in the secondstep with the mixture obtained in the first step, when it isincorporated as the fibrous material (I) in the form of sheet to becomposited with the proton-conducting membrane.

[0113] The 63^(rd) aspect of the present invention is a fuel cell whichuses the proton conducting membrane of one of the first to 23^(rd)aspects.

BRIEF DESCRIPTION OF THE DRAWING

[0114]FIG. 1 is a voltage-current curve for illustrating the outputproduced by the fuel cell of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0115] The present invention is described in detail for each aspect.

[0116] 1. Structure of the Proton-Conducting Membrane

[0117] The proton-conducting membrane of the present invention has acrosslinked structure. By contrast, the conventional proton-conductingmembrane of fluorine-based resin or polymeric material with an aromaticmolecular structure in the main chain has no such a structure. As aresult, a proton-conducting membrane of the conventional polymericmaterial is structurally changed significantly at high temperature dueto the creep phenomenon or the like. Therefore, a fuel cell which usesthe membrane exposed to high temperature is operationally unstable.

[0118] For example, a Nafion^(R) (Du Pont) membrane, which representsthe fluorine-based resin membranes, is greatly swollen under a wetcondition to lose strength, although strong and flexible under a drycondition. A membrane which shows a significantly increased size under awet condition from that under a dry condition causes various problems,e.g., difficulty in making a membrane-electrode assembly (sometimesreferred to as MEA), and possible failure of the membrane or MEA whilethe fuel cell is working, because the membrane invariably expands orcontracts under changed temperature and humidity conditions within thefuel cell as a result of the changed operational conditions. Moreover,failure of the membrane is possibly caused not only by the dimensionalchange but also by a differential pressure produced within the cell,because of its decreased strength under a wet condition.

[0119] When exposed to high temperature, e.g., around 150° C., forextended periods, the membrane will become jelly-like and useless for afuel cell, because it greatly loses its strength and is itself broken.Even at around 120° C., the creep phenomenon occurs to modify it into aswollen state and greatly decrease its strength. Once modified, it willbecome hard and fragile when dried under changed operating conditions ofthe fuel cell. This possibly causes its failure and cracking, andeventually failure of the membrane-electrode assembly itself. Thesephenomena can similarly occur in the case of the membrane with anaromatic molecular structure in the main chain.

[0120] However, these problems can be solved by introducing acrosslinked structure in the membrane. In other words, the membranehaving a crosslinked structure at a sufficient crosslinking density willno longer undergo significant changes in size and hence in strengthunder dry and wet conditional cycles.

[0121] The crosslinked structure can be formed by incorporating, e.g.,epoxy, crosslinkable acrylic, melamine or unsaturated polyester resin.However, such a crosslinked structure is not sufficiently stable forextended periods for a fuel cell membrane, which is exposed to hightemperature and humidity under a strongly acidic condition resultingfrom the presence of proton.

[0122] On the other hand, a metal-oxygen bond is stable under stronglyacidic and high temperature/humidity conditions, and can be suitablyused for the crosslinked structure within a fuel cell membrane. Thesebonds include silicon-oxygen, aluminum-oxygen, titanium-oxygen andzirconium oxygen bonds, of which silicon-oxygen bond is particularlypreferable because it can be easily obtained and is inexpensive.

[0123] The crosslinked structure for the proton-conducting membrane ofthe present invention is mainly formed by silicon-oxygen bond. However,the other metal-oxygen bonds may be used, so long as they make nosacrifice of the cost and easiness of the production process. Moreover,silicon-oxygen bond may be used in combination with phosphorus-oxygen orboron-oxygen bond. When silicon-oxygen bond is used in combination withanother metal-oxygen bond, the ratio of silicon to the other metal inthe crosslinked structure is generally 50% by mol or more per 100% bymol of the total metallic atoms, although not determined sweepingly.

[0124] 2. Organic/inorganic hybrid structure (A)

[0125] An organic/inorganic hybrid structure (A) covalently bonded to 2or more silicon-oxygen crosslinks and containing carbon atom is used asthe basic crosslinked structure for the proton-conducting membrane ofthe present invention.

[0126] Proton-conducting membranes including a silicon-oxygen bond havebeen studied for fuel cells. Each of these membranes is produced by thesol-gel process involving hydrolysis/condensation using tetraethoxy ortetramethoxy silane as the starting material. For example, the processproposed by Minami et al., described earlier, gives a proton-conductingmembrane durable to high temperature (Solid State Ionics, 74 (1994), P.105).

[0127] However, the sol-gel process involving only an inorganic materialgives a glassy, hard, fragile membrane. A proton-conducting membrane isgenerally produced to have a thickness of 100 μm or less. A glassy,fragile membrane 100 μm or less in thickness will be easily cracked,when it is assembled with an electrode, incorporated in a fuel cell orworking in the fuel cell. It is therefore difficult to use such amembrane for a fuel cell. It is in itself difficult to produce adefect-free membrane having a sufficient size (e.g., 10 square inches)to be assembled into a fuel cell.

[0128] Compositing with a flexible structure is effective for overcomingthese fragility-related problems. Some fuel cell membranes include acrosslink of oxygen with a mono- or di-alkyl-substituted silicon,produced from methyltriethoxy silane or the like as the startingcompound having an alkyl-substituted, hydrolyzable silyl group, notingheat resistance of an inorganic material and attempting to impartflexibility of an organic material to the membrane.

[0129] For example, Poinsignon et al. propose a process for condensingbenzyltriethoxysilane, n-hexyltriethoxysilane or triethoxysilane(Electrochimica Acta, 37 (1992), P.1615). It is reported that themembrane produced by this process, although having flexibility to someextent, is soluble in water, when benzyltriethoxysilane, for example, issulfonated, because of greatly reduced substantial crosslinking density.They attempt to further incorporate the crosslinked structure to preventthe above problem. However, the resultant membrane becomes fragile, andfails to achieve originally intended flexibility. In other words, it isimpossible to produce a flexible membrane when an alkyl- oraryl-substituted silicon-oxygen crosslinked structure has the crosslinkonly at one terminal.

[0130] On the other hand, the proton-conducting membrane of the presentinvention includes the organic/inorganic hybrid structure (A), describedabove, as the basic crosslinked structure. The membrane including such acompound can have adjusted membrane properties, beginning withflexibility, by designing the molecular structure between thecrosslinked structures. Fuel gas barrier capacity as one of theimportant properties for a fuel cell membrane can be adjusted bycontrolling crosslinking density and structure.

[0131] The concrete structure of the organic/inorganic hybrid structure(A) is described.

[0132] First, the structures having 3 or more crosslinked sites aredescribed. These are some of the examples which can be easily obtainedcommercially or synthesized, and the present invention is by no meanslimited them. The term “crosslinkable silyl group” described herein is asilicon oxide group bonded to a crosslinking group via a covalent bond.For example, those having 3 crosslinked sites include 1,3,5-tris(crosslinkable silylethyl)-2,4,6-trimethyl benzene, tris(p-crosslinkable silylpropylphenyl)amine and tris (p-crosslinkablesilylpropyl)isocyanurate. The hydrolyzable silyl compounds as thestarting materials for these compounds are supplied by Gelest, Inc.

[0133] Moreover, for 1,2,4-tri(crosslinkable silylethyl)cyclohexane,tri(crosslinkable silylpropyl)amine and the like, the crosslinkable,starting compounds can be produced by hydrosilylation oftrialkoxysilane, dialkoxyalkylsilane or monoalkoxydialkylsilane with acommercial reagent, e.g., 1,2,4-trivinyl cyclohexane, triallylamine orthe like (e.g., those supplied by Aldrich) in the presence ofchloroplatinic acid as a catalyst.

[0134] The crosslinkable, starting compounds can be produced by thereaction with, e.g., 3-triethoxysilylpropylisocyanate or the like forthose having 3 hydroxyl or amino groups in the molecular structure, andby the reaction with, e.g., 3-triethoxysilylpropylamine for those havinga reactive group, e.g., isocyanate, in the molecular structure.

[0135] Next, those having 4 crosslinked sites includetetrakis(crosslinkable silylpropyl)silane and tetrakis(crosslinkablesilylethyl)silane, for which the starting materials can be easilyproduced by the similar hydrosilylation of correspondingtetraallylsilane (Gelest, Inc.) and tetravinylsilane (Aldrich) as thecommercial products. In other words, it is sufficiently possible tosynthesize the other structures having 3 or 4 crosslinked sites.Moreover, those having 5 or more crosslinked sites can be alsosynthesized by use of the so-called dendrimer or the like as thestarting material. The still other examples include those structuresfrom a starting material having a hydrolyzable silyl group in thepolymeric side chain. Those having 2 or more hydrolyzable silyl groupsin the straight-chain or cyclic siloxane side chain may be also used.

[0136] Those having 2 crosslinked sites in the molecular chain includethe structures having a crosslinkable silyl group in the middle of themolecular chain. Those represented by the general formula (1) are morepreferable for their availability:

[0137] (wherein, X is an —O— bond or OH group involved in thecrosslinking; R¹ is a carbon-containing group of 1 to 50 carbon atoms;R² is methyl, ethyl, propyl or phenyl group; and “n” is an integer of 0,1 or 2).

[0138] Some of the crosslinkable precursors for these structures arecommercially available and directly used. They can be synthesized fromthose precursors having an unsaturated bond by hydrosilylation of thecorresponding silyl compounds. They can be similarly synthesized whenthe precursors have hydroxyl, amino group or the like.

[0139] Popall et al. propose a similar system, which is a coatingmaterial with a silicon-oxygen crosslink at one terminal and anothercrosslink with epoxy or methacrylic group at the other terminal(Electrochimica Acta, 43 (1998), P.1301). These crosslinked structures,however, have the ether or ester bond amenable to hydrolysis under thehigh temperature/humidity and strongly acidic conditions produced whilethe fuel cell is working, the former bond being produced by the epoxycrosslinking and the latter left by the methacrylic bond crosslinking.The crosslinked structure should be highly resistant to acid and heat,e.g., that formed by a crosslinking group such as Si—O, like the oneused for the present invention, to be used for a fuel cell. The mixedsystem of organic and inorganic crosslinks proposed by Popall et al. isintended for a patterning material but not for a fuel cell membrane. Theone for a patterning material is similar to that for a fuel cell, buttechnically quite different, because of different objects and functions.

[0140] X in the general formula (1) is a bond involved in thecrosslinking or silanol group which can be involved in the crosslinking.Number of the bonds or groups in the structure is 3, 2 or 1 (i.e., “n”is 0, 1 or 2).

[0141] R¹ is a carbon-containing group for controlling membraneproperties, e.g., flexibility. The membrane will be hard and fragilewhen it contains no carbon atom. It will be insufficiently crosslinkedand sufficient heat resistance will be no longer expected, when thechain has more than 50 carbon atoms.

[0142] The preferred embodiments of R¹ include hydrocarbon groups. R¹may contain a hetero atom, but may be decomposed when exposed to an acidor heat. By contrast, a hydrocarbon compound is resistant to an acid,and very stable. The hydrocarbons include alkylene andaromatic-containing chains.

[0143] Of these, particularly preferable ones include straight-chainmolecular chains composed of polymethylene chain free of branch or thelike. They are represented, e.g., by the general formula (3):

[0144] (wherein, “n” is an integer of 1 to 30).

[0145] When R¹ is branched, for example, the methine hydrogen in thebranch may be pulled out by an active radical or the like produced whilethe fuel cell is working to cut the bond which connects the crosslinksto each other. When it contains an aromatic compound, mainly the benzylsite may become an active site to trigger decomposition or anotherreaction, to possibly deteriorate stability of the membrane. R¹containing a hydrocarbon compound having an aromatic ring is more stablethan that having a hetero atom, but possibility of decomposition cannotbe ruled out when the fuel cell works for extended periods.

[0146] By contrast, when R¹ is a straight-chain polymethylene chain, itmakes the structure stable to attacks by an acid, radical or the like.Such a structure is suitable for a heat-resistant fuel cell membrane.The straight-chain polymethylene chain is not only stable but also offlexible structure, and can impart adequate flexibility to the membrane.Therefore, the membrane can be adjusted for denseness, the adjustmentmainly achieved by molecular length of the polymethylene chain.

[0147] Various types of bis (hydrolyzable silyl) polymethylene are knownto serve as the starting compounds for introducing the Si—O crosslinksinto the polymethylene chain at both terminals. The polymethylene ofethylene, hexamethylene, octamethylene, and nonamethylene arecommercialized by Gelest, Inc. Moreover, the starting compounds with R¹corresponding to tetramethylene, decamethylene, tetradecamethylene,hexadecamethylene or docosamethylene can be easily produced byhydrosilylation of the corresponding compound with unsaturated bonds atboth terminals, e.g., 1,3-butadiene, 1,9-decadiene, 1,13-tetradecadiene,1,15-hexadecadiene or 1,21-docosadiene. Any polymethylene chain can besynthesized, so long as it has 30 carbon atoms or less.

[0148] The polymethylene having a molecular length of 1 to 30 can givethe membrane which satisfies all of the properties of heat resistance,flexibility and fuel gas barrier characteristics. The membrane tends tobe more flexible as methylene molecular chain length increases, andtougher as it decreases, although not sweepingly generalized, becausethese properties are also affected by number of the crosslinking groups.Of these polymethylenes, those having the structure with 8 methylenesconnected in series, represented by the general formula (4), are morepreferable for their availability:

[0149] (wherein, X is an —O— bond or OH group involved in thecrosslinking; and “n” is an integer of 0, 1 or 2).

[0150] Those polymethylene structures with the Si—O crosslinks at bothterminals are very stable and useful as the basic crosslinked structuresfor proton-conducting membranes for fuel cells.

[0151] Like a polymethylene, a siloxane compound is also useful for R¹in the general formula (1), because it is high in resistance to heat andacid and can give a flexible membrane. The siloxane compound has theSi—O bond in the main chain and an organic group, e.g., alkyl group, inthe side chain.

[0152] The siloxane compound may have a branched or cyclic structure, orthe like, but is particularly preferably of straight-chain structure forits high flexibility. These straight-chain siloxane compounds have astructure represented by the general formula (5):

[0153] (wherein, R⁵ and R⁶ are each methyl, ethyl, propyl or phenylgroup, which may be the same or different; and “n” is an integer of 1 to20).

[0154] R⁵ and R⁶ are generally methyl group for the dimethyl siloxanestructure. However, those having ethyl, propyl or phenyl group toincrease solubility can be also suitably used. These siloxane compoundscan be produced from those having alkoxysilyl, silanol, halogenatedsilyl or silanolate group at the corresponding siloxane terminal as thestarting compounds, which are commercialized by, e.g., Gelest, Inc.

[0155] The siloxane compound with silanol group at the terminal may bereacted with tetraethoxysilane, tetraacetoxysilane or hydrolyzablemetallic compound of Ti, Zr, Al or the like, to be more crosslinkablebeforehand.

[0156] The structure of the straight-chain siloxane with the Si—Ocrosslink at the terminal is very stable and useful as the basiccrosslinked structure for proton-conducting membranes for fuel cells.

[0157] The organic/inorganic hybrid structure (A) maybe composed of 2 ormore structure types. For example, it may be a mixture oforganic/inorganic hybrid structure having a hydrocarbon compound andthat having a siloxane compound. Such a mixture can be adjusted forcrosslinking density or the like by the composition, and hence formembrane flexibility, gas barrier characteristics or the like. Themembrane properties can be also adjusted by mixing organic/inorganichybrid structures of different organic chain length, crosslinking groupnumber, substituent type or the like.

[0158] 3. Structure (B) Containing Acid Group

[0159] A proton-conducting membrane for fuel cells is generally requiredto efficiently conducting proton. Proton-conducting efficiency of themembrane basically depends on proton concentration in the membrane,content of the conducting medium (e.g., water) and proton mobility,although varying to some extent depending on the conducting mechanisminvolved. In other words, it is preferable that proton is present at ahigh concentration in the membrane. For proton to be present at a highconcentration in the membrane, it is necessary to distribute acid groupsas much as possible in the membrane.

[0160] When the acid group is extracted and released out of the membranein the presence of water supplied to the fuel cell or of water or thelike produced while the fuel cell is working, proton concentration inthe membrane decreases to deteriorate its proton conductivity.Therefore, some measures are preferably taken to securely hold the acidin the membrane stably for extended periods, e.g., by covalently bondingthe acid instead of ionic interactions. Nafion^(R), for example,contains sulfonic acid via covalent bond, and it is known that thesulfonic acid group itself is stably kept in the membrane for extendedperiods, although the membrane may be deteriorated by the creepphenomenon.

[0161] In the proton-conducting membrane of the present invention, acompound containing an acid group is bound to the silicon-oxygencrosslink via covalent bond. More specifically, proton conductivity ofthe membrane can be stably secured for extended periods by stronglybinding the acid to the organic/inorganic hybrid structure (A) as thebasic structure for the membrane. The present invention is particularlycharacterized by combining the acid-containing structure (B) with theorganic/inorganic hybrid structure (A), which can impart heat resistanceand flexibility to the membrane. As a result, the membrane is resistantto heat and good in membrane properties, e.g., flexibility, and stablyholds the acid.

[0162] The structure (B) containing an acid group is not limited, solong as it has an acid group and is bound to the crosslinked structurein the membrane via the Si—O bond.

[0163] However, it has preferably a structure by the general formula(2):

[0164] (wherein, X is an —O— bond or OH group involved in thecrosslinking; R³ is a molecular chain group having at least one acidgroup; R⁴ is methyl, ethyl, propyl or phenyl group; and “m” is aninteger of 0, 1 or 2).

[0165] R³ has at least one acid group and is bound to the crosslinkinggroup via a covalent bond. Various acid groups are useful for thepresent invention. For example, they include sulfonic, phosphonic,carboxylic, sulfuric, phosphoric and boric acid, of which sulfonic acidis particularly preferable, because it has a low pKa value, can secureproton in the membrane at a sufficiently high concentration and isthermally stable.

[0166] When the acid group is sulfonic acid, R³ preferably has astructure represented by the general formula (6):

[0167] (wherein, “n” is an integer of 1 to 20).

[0168] The structure between sulfonic acid and crosslinked structure isnot limited, but should be excellent in resistance to heat, acid andoxidation among others for the object of the present invention. Thepolymethylene chain represented by the general formula (6) is one of thestructures which satisfy the above requirements. In the structurerepresented by the general formula (6), the polymethylene chain is notbranched, and the sulfonic acid group is present at the terminal of thepolymethylene chain.

[0169] When the polymethylene chain is branched, the methine structurein the branch is amenable to oxidation or radical reaction, with theresult that sulfonic acid may be released out of the membrane. Whensulfonic acid is present in the polymethylene chain not at the terminalbut in the middle, the portion bound to sulfonic acid may bemethine-structured, with the result that sulfonic acid may be similarlyeliminated/released by oxidation or the like.

[0170] Moreover, the structure in which sulfonic acid is bound to thecrosslinked structure is preferably free of aromatic ring. An aromaticring is easily sulfonated. For example, Poinsignon et al. prepare astructure in which sulfonic acid is directly incorporated in the benzenering by forming a crosslinked structure of benzyl trialkoxysilanebeforehand and then sulfonating the structure (Electrochimica Acta, 37(1992), P.1615, cited before). However, direct sulfonation of anaromatic compound involves a disadvantage that sulfonic acid is easilyeliminated from the structure, although is easily synthesized. In otherwords, such a structure is easily desulfonated when exposed to hightemperature/humidity conditions associated with operation of a fuelcell, for which the membrane of the present invention is developed, tolose conductivity. It is known that sulfone group is prepared from anaromatic ring via several methylene chains by addition of an adequatecompound, e.g., 1,3-propane sulfone, instead of direct sulfonation(Ogata et al., Polymer Preprint, Japan, 46 (1997), P. 1867). In thiscase, however, methylene adjacent to the aromatic ring may become anactive site (the so-called benzyl site) to cause elimination of the acidby decomposition or the like originating from the benzyl site. This cangreatly reduce proton conductivity at a high possibility. Therefore, itis not suitable for the acid-bound structure for the present invention.

[0171] Based on the above considerations, the optimum structure of thestructure (B) containing an acid group for the present invention isrepresented by the general formula (6), where the one having a siliconatom directly bound to sulfone group (i.e., “n” is 0) is amenable tohydrolysis and hence unsuitable for the present invention. On the otherhand, the one having “n” larger than 20 is also undesirable, because itgives a membrane of insufficient crosslinking density. Therefore, “n” isin the range from 1 to 20, preferably 1 to 12.

[0172] Of these structures, the one having “n” of 3 is particularlypreferable because it is easily obtained;3-trihydroxysilylpropylsulfonic acid, which can be used as the startingmaterial, is commercialized by Gelest, Inc., and the synthesis processusing allyl bromide as the starting compound is already established.

[0173] 4. Ratio of the Organic/Inorganic Hybrid Structure (A) to theAcid-Containing Structure (B)

[0174] The ratio of the organic/inorganic hybrid structure (A) to theacid-containing structure (B) is not limited, so long as it is in therange defined for satisfying the required heat resistance, flexibilityand proton conductivity of the membrane. Generally speaking, at aninsufficient content of the organic/inorganic hybrid structure (A),sufficient flexibility and heat resistance of the membrane cannot beachieved. Moreover, the membrane may not be self-sustaining. At aninsufficient content of the acid-containing structure (B), on the otherhand, the membrane may have a very low proton conductivity.

[0175] The (A)/(B) ratio is normally in a range from 1:9 to 9:1,although varying depending on structure of each structure and process bywhich the membrane is produced.

[0176] 5. Fibrous Material (I)

[0177] The proton-conducting membrane of the present invention has athree-dimensional silicon-oxygen structure. It is resistant to heat,swells and contracts to a limited extent with changed humidity, and istough. However, it may be fragile when made into a thin film, and may beincorporated with a reinforcing agent. The reinforcing agent useful forthe present invention is not limited, so long as it does not preventproton conduction and exhibits an effect of reinforcing the membrane. Itmay be in the fibrous, fibril or porous membrane form. The reinforcingagents are represented by the fibrous material (I).

[0178] The fibrous material (I) should be resistant to high temperatureand acid concentration within a fuel cell. Those suitably used for thepresent invention include fluorine resin represented bypolytetrafluoroethylene, cyclic polyolefin resin, high-molecular-weightpolyolefin and inorganic materials, e.g., glass as the materialsresistant to these severe environments.

[0179] Of these, fluorine resin is particularly preferable as areinforcing agent for the proton-conducting membrane of the presentinvention for its high chemical stability. Those fluorine resins usefulfor the fibrous material for the present invention include TomoegawaPaper's P-50. Moreover, porous membranes, e.g., Advantech's H₀₂₀A142Cand Nihon Millipore's membrane filter JG, are also useful. Thesefluorine materials may not be always adhesive securely to aproton-conducting material. Therefore, they may be surface treated, asrequired, by a wet process with a silane coupling agent, or dry process,e.g., corona or plasma treatment.

[0180] The glass fiber material is highly adhesive to theorganic/inorganic hybrid structure (A) and acid-containing structure(B). Silanol group, when left on the glass fiber surfaces, can reactwith the structures (A) and (B), to make the glass fibers highly fastadhesive to the structures (A) and (B). Therefore, it brings a desirableeffect for the present invention.

[0181] Alkali- or acid-resistant glass is more preferable than common Eglass for the fibrous material, because it is more resistant to acid,which is present at a high concentration within a fuel cell.

[0182] Glass is an inorganic material generally composed of SiO₂, B₂O₃,P₂O₅, Al₂O₃, among others, as the major ingredients, and normallyincorporated with an alkali component, e.g., Na₂O or K₂O, to decreasesoftening temperature.

[0183] Alkali-resistant glass may be incorporated with a Ca₂O componentto prevent flow of alkali, and means glass having a chemical formularepresented by Na₂O.ZrO₂ (TiO₂)-SiO₂. It is accepted that increasingZrO₂ content increases resistance to alkali.

[0184] Acid-resistant glass preferably contains an alkali component,e.g., Na₂O or K₂O, at a lower content. For example, quartz glass mainlycomposed of SiO₂, or borosilicate glass mainly composed of SiO₂, B₂O₃and the like are suitably used.

[0185] The fibrous material (I) may morphologically fall into shortfibrous material (J) and long fibrous material (K). Each is described indetail.

[0186] The present invention may be incorporated with a short fibrousmaterial (J) only, long fibrous material (K) only or combination thereofas the fibrous material (I).

[0187] The fibrous material (I), when composed of glass or anotherinorganic material, can be treated at high temperature or by oxidationto activate the surface. This surface treatment is preferable for thepresent invention, because it improves its adhesion to theorganic/inorganic hybrid structure (A) and acid-containing structure(B).

[0188] The fibrous material may be also treated with a silane couplingagent, which can be selected from the commercial products inconsideration of its adhesion to the organic/inorganic hybrid structure(A) and acid-containing structure (B).

[0189] The suitable silane coupling agent contains an acid group and canimpart proton conductivity to the fibrous material (I) surface. Thesesilane coupling agents include 3-trihydroxysilylpropanesulfonic acid.The fibrous material (I) may be also treated for silane coupling with asulfur-containing compound, e.g., 3-mercaptopropyltriethoxysilane andthen for oxidation, or treated with a halogenated silane coupling agent,e.g., 3-bromopropyltriethoxysilane and then treated to substitute thehalogen group by sulfurous acid. The other common treatment methods witha silane coupling agent include drying and/or baking a mixture ofalkoxysilane or halogenated silane and the fibrous material (I) afterhydrolyzing the former to be compatible with the latter.

[0190] 5.1 Short Fibrous Material (J)

[0191] The short fibrous material (J) is 1 to 1000 μm long, preferably 5to 100 μm long.

[0192] The short fibrous material (J) is preferably in the form of shortglass fibers (M) or whiskers (L). The short glass fibers (M) can beproduced from glass fibers by an adequate method, e.g., crushing.

[0193] The whiskers (L) are fine fibers of crystalline structure, andused for reinforcing the membrane to prevent cracking. The whiskerspreferably have dimensions of diameter: 0.1 to 3 μm, length: 1 to 20 μmand aspect ratio: 5 to 100, more preferably 10 to 50. When excessivelyfine, the whiskers will agglomerate with each other, and difficult tohandle. When excessively coarse, they may not fully exhibit thereinforcing effect. The materials for the whiskers (L) include boroncarbide, silicon carbide, alumina, aluminum borate, silicon nitride andK₂O.6TiO₂. The reinforcing agent preferably has a functional group(e.g., OH group), on the surface, which can trigger the silane couplingreaction, to have improved adhesion to the hardenable material whichforms the silicon-oxygen bond. The whiskers (L) can have an activatedsurface when treated for oxidation on the surface.

[0194] When excessively incorporated, the whiskers (L) may not bedispersed sufficiently to possibly cause excessive permeation of thegas, and may decrease conductivity of the membrane. When incorporatedinsufficiently, they may not fully exhibit their effect. Therefore, theyare incorporated at 1 to 75 parts by weight per 100 parts by weight ofthe organic/inorganic hybrid structure (A) and acid-containing structure(B) totaled.

[0195] The most simple method to incorporate the short fibrous material(J) into the proton-conducting membrane is mixing it with theorganic/inorganic hybrid structure (A) and acid-containing structure (B)while the starting solution of these structures are prepared. They arepreferably mixed by a homogenizer or ball mill, to strongly agitate themto prevent agglomeration. The mixture is preferably used immediatelyafter it is prepared to prevent separation, or else treated to have acertain viscosity to prevent the mixture from being easily separated.

[0196] 5.2 Long Fibrous Material (K)

[0197] The long fibrous material (K) is preferably 10 mm long or moreand continuous, viewed from the reinforcing effect.

[0198] The preferable materials for the long fibrous material (K)include fluorine resin represented by polytetrafluoroethylene, andglass, in particular glass, for their excellent adhesion to thecrosslinked structure (A).

[0199] The glass fibers are preferably, in particular, 10 mm long ormore and continuous, viewed from the reinforcing effect. They are alsopreferably 3 to 20 μm in diameter, more preferably 9 to 13 μm. Whenfiner than 3 μm, they tend to be scattered into air. Moreover, it isaccepted that they can easily enter the blood tubes to injure humanhealth. The fibers having a diameter larger than 20 μm are extremelystimulus to the human skin, and are not well distributed in thepaper-making step, which is later described, to cause an unevendistribution.

[0200] The short glass fibers can be well dispersed when mixed andagitated with the starting materials for the structures (A) and (B).However, the long glass fibers may not be evenly distributed in theproton-conducting membrane of the present invention, unless they aredispersed beforehand in the form of thin film. The glass fiber forms aredescribed in detail.

[0201] The proton-conducting membrane of the present invention isnormally 10 to 300 μm thick, preferably 30 to 100 μm thick.

[0202] The membrane thinner than 10 μm may be insufficiently durable andtends to suffer defects, e.g., pinholes. The one thicker than 300 μm hasexcessively high resistance to conduction, and is also unsuitable forfuel cells as an electrolytic membrane.

[0203] The glass fibers for the proton-conducting membrane fall into 3types in consideration of the limitation of membrane thickness; wovenfabric, nonwoven fabric and glass fiber paper made by a paper-makingprocess. Of these, woven and non-woven fabric can be selected, when theglass fibers are used as the continuous ones. The fabric is preferablyof a type which can use continuous fibers in order to exhibit asufficient strength in the form of thin film.

[0204] The woven fabrics may be square, diagonal, Turkish satin, gauzeelastic, and leno weaves, among others, of which a square weave isparticularly preferable for the present invention to prevent elongationof the membrane. The square weave, is preferably not excessively tightto reduce its effect of blocking proton conductance in the membrane,because the ion conductance passages may be blocked in an excessivelytight membrane. A fairly coarse weave, e.g., that of yarns thinned outon every other yarn, is preferable. Such a weave is referred to asthirled, square weave in the present invention.

[0205] The square weave structure is defined by yarn count and density(pitch), among others. The following structure is preferable for theproton conducting membrane of the present invention.

[0206] The yarn is a bundle of 50 to 1000 twisted glass fibers, and itscount is represented by Tex having a unit of mass (g) per 1000 m. Itscount is preferably 3 to 50 Tex. The finer yarn is more easily cut inthe production process, although showing better properties. The yarnthicker than 50 Tex is difficult to make the base on the thin filmsuitable for the proton conducting membrane.

[0207] Weave density, which is referred to as count density or pitch,means number of the yarns per 25 mm width. The weave of low densitycannot exhibit the reinforcing effect sufficiently. Conversely, weavedensity is limited by fineness of the yarn. The weave preferably has adensity of 40 to 200 yarns/25 mm width.

[0208] Thickness of the square weave is essentially determined by theabove specifications, 20 to 100 μm based on the above specifications.

[0209] Density (mass per unit area), which is related to thickness, isnormally 10 to 50 g/m², preferably 15 to 25 g/m² for the same reasonsdescribed above.

[0210] The long fibrous material (K) cannot directly form a uniformcomposite structure, even when mixed beforehand with the startingmaterials for the organic/inorganic hybrid structure (A) andacid-containing structure (B), unlike the short fibrous material (J).Therefore, it preferably has a specific shape (sheet) before it isincorporated.

[0211] The long fibrous material (K), to be incorporated to form acomposite structure, is preferably loaded with a starting mixture of theorganic/inorganic hybrid structure (A) and acid-containing structure(B). It may be loaded by pressing or the liquid, starting mixture on thesheet, or rolling. The loading method can be optionally selected fromthe known, simple ones.

[0212] The loading methods are described above for glass fibers.However, the similar methods are applicable to the other fibrousmaterials, e.g., those composed of fluorine resin, cyclopolyolefin resinor high-molecular-weight polyolefin resin.

[0213] 6. Other Additives

[0214] The proton conducting membrane of the present invention canexhibit its intended performance so long as it satisfies therequirements of simultaneously containing (a) an organic/inorganichybrid structure (A) covalently bonded to 2 or more silicon-oxygencrosslinks and having a carbon atom, and (b) an acid containingstructure (B) having an acid group, covalently bonded to asilicon-oxygen crosslink and having an acidic group. However, it may befurther incorporated with another additive within limits not harmful toits performance.

[0215] These additives include the following i) to iv):

[0216] i) hydrophilic, polymeric compound, to impart hydrophilicity tothe membrane,

[0217] ii) finely powdered metallic oxide, such as silica, to impartwater retentivity to the membrane,

[0218] iii) reinforcing agent composed of glass or the like of fibril orfibrous structure, to be used as the membrane base, the reinforcingagent is described earlier, and

[0219] iv) auxiliary acid, or salt, ester or amide structure to improveconductivity of the membrane.

[0220] The membrane may be also dispersed with a catalyst, e.g.,platinum, although necessity therefore varies depending on electrodestructure or the like.

[0221] Content of these additives is not limited so long as it is notharmful to performance of the membrane. However, content of the totaladditives is preferably 50% or less by weight based on the totalmembrane weight, although not sweepingly generalized, because contentnot harmful to the performance greatly varies additive by additive.

[0222] 7. Method for Producing the Proton Conducting Membrane

[0223] The proton conducting membrane of the present invention can beproduced by various methods, and the method is not limited. Forexamples, the major methods include the following four types(hereinafter referred to the first to fourth methods):

[0224] 1) First Method

[0225] First step: Preparing a mixture containing an organic/inorganichybrid, crosslinkable compound (C) and compound (D), the former having 2or more crosslinkable silyl groups and carbon atoms each being bonded tothe silyl group via the covalent bond and the latter having acrosslinkable silyl group and acid group.

[0226] Second step: Forming the above mixture into a film.

[0227] Third step: Hydrolyzing/condensing or only condensing thehydrolyzable silyl group contained in the mixture formed into the filmto form a crosslinked structure.

[0228] 2) Second Method

[0229] First step: Preparing a mixture containing an organic/inorganichybrid, crosslinkable compound (C) and compound (E), the former having 2or more crosslinkable silyl groups and carbon atoms each being bonded tothe silyl group via the covalent bond and the latter having acrosslinkable silyl group and mercapto group.

[0230] Second step: Forming the above mixture into a film.

[0231] Third step: Hydrolyzing and condensing the hydrolyzable silylgroup contained in the mixture formed into the film to form acrosslinked structure.

[0232] Fourth step: Oxidizing the mercapto group in the crosslinkedstructure obtained in the third step into sulfonic acid.

[0233] 3) Third Method

[0234] First step: Preparing a mixture containing an organic/inorganichybrid, crosslinkable compound (C) and compound (F), the former having 2or more crosslinkable silyl groups and carbon atoms each being bonded tothe silyl group via the covalent bond and the latter having acrosslinkable silyl group and polysulfide group.

[0235] Second step: Forming the above mixture into a film.

[0236] Third step: Hydrolyzing and condensing the hydrolyzable silylgroup contained in the mixture formed into the film to form acrosslinked structure.

[0237] Fourth step: Oxidizing the polysulfide group in the crosslinkedstructure obtained in the third step into sulfonic acid.

[0238] 4) Fourth Method

[0239] First step: Preparing a mixture containing an organic/inorganichybrid, crosslinkable compound (C) and compound (H), the former having 2or more crosslinkable silyl groups and carbon atoms each being bonded tothe silyl group via the covalent bond and the latter having acrosslinkable silyl group and halogen group.

[0240] Second step: Forming the above mixture into a film.

[0241] Third step: Hydrolyzing and condensing the hydrolyzable silylgroup contained in the mixture formed into the film to form acrosslinked structure.

[0242] Fourth step: Substituting the halogen group in the crosslinkedstructure obtained in the third step with sulfonic acid group.

[0243] 7.1 First Method

[0244] This method comprises, as described above, the first step ofpreparing a mixture containing an organic/inorganic hybrid,crosslinkable compound (C) and compound (D), the former having 2 or morecrosslinkable silyl groups and carbon atoms each being bonded to thesilyl group via the covalent bond and the latter having a crosslinkablesilyl group and acid group, second step of forming the above mixtureinto a film, and third step of hydrolyzing/condensing or only condensingthe hydrolyzable silyl group contained in the mixture formed into thefilm to form a crosslinked structure.

[0245] It is described in more detail for the starting materials andtreatment conditions, among others, for these steps orderly.

[0246] The first method of the present invention first prepares amixture containing an organic/inorganic hybrid, crosslinkable compound(C) and acid containing compound (D).

[0247] The proton conducting membrane of the present invention shouldsatisfy the requirements of simultaneously containing (a) anorganic/inorganic hybrid structure (A) covalently bonded to 2 or moresilicon-oxygen crosslinks and having a carbon atom, and (b) an acidcontaining structure (B) having an acid group, covalently bonded to asilicon-oxygen crosslink and having an acidic group. Therefore, thestarting materials each corresponding to the organic/inorganic hybridstructure (A) or acid containing structure (B) are used to form thecrosslinked structure.

[0248] A hydrolyzable silyl group is one of the crosslinking precursorsfor forming the crosslinked structure. The hydrolyzable silyl group asthe crosslinking precursor forms the crosslink composed of the Si—O bondby hydrolysis and subsequent condensation. This process is known as thesol-gel process.

[0249] The hydrolyzable silyl groups include alkoxysilyl groups withalkoxy group (e.g., methoxy, ethoxy, propoxy or phenoxy) directly bondedto the silicon atom, halogenated silyl groups with a halogen (e.g.,chlorine) bonded to the silicon atom, and carboxylated silyl groups(e.g., acetoxy). Moreover, silanol and silanolate groups, which arehydrolyzed beforehand, may be also used. In this case, hydrolysis is nolonger necessary and the third step may only involve condensing.

[0250] It is important for the present invention to use anorganic/inorganic hybrid, crosslinkable compound (C) having 2 or morecrosslinkable silyl groups and carbon atoms each being bonded to thesilyl group via the covalent bond, in order to form theorganic/inorganic hybrid structure (A).

[0251] The crosslinkable compounds having 3 or more crosslinked sitesinclude 1,3,5-tris(trichlorosilylethyl)-2,4,6-trimethylbenzene,tris(p-trichlorosilylpropylphenyl)amine andtris(3-trimethoxysilylpropyl)isocyanurate. They are commercialized byGelest, Inc., and can be directly used as the crosslinkable compounds.Moreover, 1,2,4-trivinylcyclohexane and triallylamine (bothcommercialized by Aldrich) can be treated for hydrosilylation withtrialkoxysilane, dialkoxyalkylsilane or monoalkoxydialkylsilane toproduce the corresponding starting compounds. The starting compounds canbe also produced by the reaction with, e.g.,3-triethoxysilylpropylisocyanate or the like for the compounds havinghydroxyl or amino group in the molecular structure, and by the reactionwith, e.g., 3-triethoxysilylpropylamine for those having a reactivegroup, e.g., isocyanate, in the molecular structure.

[0252] The crosslinkable compounds having 4 crosslinked sites can beeasily produced by the hydrosilylation of tetraallylsilane (Gelest,Inc.) or tetravinyl silane (Aldrich) as the commercial products. Inother words, it is sufficiently possible to synthesize various startingcompounds having 3 or more crosslinked sites. Similarly, those having 2or more crosslinkable silyl groups in the side chain of straight-chainor cyclic siloxane or the like can be synthesized or commerciallyavailable.

[0253] For the crosslinkable compounds having 2 crosslinked sites in themolecular chain, the precursors represented by, e.g., the generalformula (7) is suitably used:

[0254] (wherein, R¹ is a carbon-containing group of 1 to 50 carbonatoms; R² is methyl, ethyl, propyl or phenyl group; R⁵ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “n” is 0, 1 or 2).

[0255] Some of the crosslinkable precursors for these structures arecommercially available and directly used. They can be synthesized fromthe precursor having the corresponding unsaturated bond, when available,by hydrosilylation of the crosslinkable silyl compound.

[0256] Many compounds with a hydrocarbon as R¹ are commerciallyavailable or easily synthesized. When it has an aromatic ring,divinylbenzene, 1,4′-divinylbiphenyl or divinylnaphthalene, for example,can be easily converted into the corresponding crosslinkable compound byhydrosilylation. Bis(trimethoxysilylethyl)benzene is commercialized byGelest, Inc.

[0257] Of these compounds, the crosslinkable compound (C) represented bythe general formula (8) ca be produced by hydrosilylation of thecorresponding diene compound:

[0258] (wherein, R² is methyl, ethyl, propyl or phenyl group; R⁵ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is an integer of 1 to 30;and “n” is 0, 1 or 2).

[0259] Some of these crosslinkable compounds, e.g.,bis(triethoxysilyl)ethane, bis(trimethoxysilyl)hexane,bis(triethoxysilyl)octane and bis(triethoxysilyl)nonane, arecommercialized by Gelest, Inc. Of these, bis(triethoxysilyl)octanehaving 8 methylene chains is readily available, and suitably used.Bis(trialkoxysilyl)octane, bis(dialkoxyalkylsilyl)octane andbis(alkoxydialkylsilyl)octane are also suitably used, because1,7-decadiene as the starting compound is easily available. Othercompounds of different chain length can be easily synthesized into thecorresponding crosslinkable compounds by hydrosilylation of thosecompounds with unsaturated bonds at both terminals, e.g., 1,3-butadiene,1,9-decadiene, 1,13-tetradecadiene, 1,15-hexadecadiene and1,21-docosadiene. Moreover, compounds of longer chain can be alsosynthesized.

[0260] For hydrosilylation of unsaturated bonds, functional group numberof the crosslinking group can be freely selected when triethoxysilane,diethoxymethylsilane or ethoxydimethylsilane is used as the hydrosilylcompound, and hence crosslinking density or the like can be finelydesigned.

[0261] A siloxane-based compound can be also used, in addition to thehydrocarbon-based compound described above, as the main skeleton of theorganic/inorganic hybrid, crosslinkable compound (C) as the startingcompound for the organic/inorganic hybrid structure (A).

[0262] A siloxane-based compound can be suitably used for aproton-conducting membrane, because of its stability to acid and heatand resistance to oxidation.

[0263] Examples of these siloxane compounds are those represented by thegeneral formula (10):

[0264] (wherein, R¹¹, R¹², R¹³ and R¹⁴ are each methyl, ethyl, propyl orphenyl group, which may be the same or different; R⁵ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is an integer of 1 to 30; and “n”is 0, 1 or 2).

[0265] These siloxane-based compounds useful for the siloxane-basedorganic/inorganic hybrid, crosslinkable compound (C) for the presentinvention include, but not limited to, the following ones.

[0266] Polydimethylsiloxane with silanol at both terminals,polydiphenylsiloxane with silanol at both terminals,polydimethylsiloxanepolydiphenylsiloxane copolymer with silanol at bothterminals, polydimethylsiloxane with chlorine at both terminals,polydimethylsiloxane with diacetoxymethyl at the terminal,polydimethylsiloxane with methoxy at the terminal, polydimethylsiloxanewith dimethoxymethylsilyl at the terminal, polydimethylsiloxane withtrimethoxysilyl at the terminal andmethoxymethylsiloxane-dimethylsiloxane copolymer, which arecommercialized by, e.g., Gelest, Inc.

[0267] Moreover, a vinyl siloxane compound with trimethoxysilane,dimethoxymetylsilane or methoxydimethylsilane added to the vinyl groupby hydrosilylation is also suitably used. These vinyl compounds includepolydimethylsiloxane with vinyl at the terminal,diphenylsiloxanedimethylsiloxane copolymer with vinyl at the terminal,polyphenylmethylsiloxane with vinyl at the terminal,polyvinylmethylsiloxane, vinyl methylsiloxane-dimethylsiloxanecopolymer, vinyl methylsiloxane-diphenylsiloxane copolymer, vinylmethylsiloxanetrifluoropropylmethylsiloxane copolymer andpolyvinylmethoxysiloxane.

[0268] Of these, a polydimethylsiloxane-based compound is particularlysuitably used, because of its availability. Thepolydimethylsiloxane-based organic/inorganic hybrid, crosslinkablecompound (C) is represented by the general formula (11):

[0269] (wherein, R⁵ is Cl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group;“m” is an integer of 1 to 30; and “n” is 0, 1 or 2).

[0270] In the first method of the present invention, it is important touse a compound (D) having a crosslinkable silyl group and acid group toform the crosslinked structure (B) containing an acid group. Thecompound (D) is not limited, so long as it contains a silyl group whichcan be bonded and acid group. For example, those compounds representedby the general formula (12) can be used:

[0271] (wherein, R³ is a molecular chain group having at least one acidgroup; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0272] Various acid groups are useful as the ones which R³ has. Forexample, they include sulfonic, phosphonic, carboxylic, sulfuric,phosphoric and boric acid, of which sulfonic acid is particularlypreferable, because it has a low pKa value, can secure proton in themembrane at a sufficiently high concentration and is thermally stable.

[0273] When the structure (B) containing an acid group has a structurerepresented by the general formula (6), the acid containing compound (D)as the starting material for the corresponding precursor has a structurerepresented by the general formula (13):

[0274] (wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; X is hydrogen, an alkalimetal, alkyl group or ammonium group; “m” is 0, 1 or 2; and “n” is aninteger of 1 to 20).

[0275] Of the starting compounds for these structures,trihydroxysilylpropylsulfonic acid for the structure having “n” of 3,commercialized by Gelest, Inc., is particularly preferable because ofits availability.

[0276] Moreover, compounds disclosed by Japanese Patent Laid-openPublication No.54-138522 (U.S. Pat. No. 4,152,165) are also suitablyused as the acid-containing compound (D).

[0277] The acid-containing structure (B) can be also produced byoxidation of a film formed using a compound having an alkoxysilyl andhalogenated alkyl group after the halogen is substituted bydithiocarbonic acid or the like; directly reacting sulfurous acid on anunsaturated bond in a film formed using a compound having an alkoxysilylgroup and the unsaturated bond; and oxidation of an unsaturated bondafter a sulfur compound, e.g., thiosulfuric acid, is added to the bond.

[0278] In the first method of the present invention, the mixing ratio ofthe organic/inorganic hybrid, crosslinkable compound (C) to theacid-containing compound (D) is not limited. In general, however, it ispreferably 9:1 to 1:9 by weight. The membrane is fragile and difficultto handle when the organic/inorganic hybrid, crosslinkable compound (C)is incorporated at below 10%. On the other hand, it cannot secure asufficient proton conductivity at its content above 90%. The above ratioalmost corresponds to the final (A)/(B) ratio in the membrane.

[0279] In the first method of the present invention, a crosslinkingagent (G) may be incorporated, as required, in addition to theorganic/inorganic hybrid, crosslinkable compound (C) and compound (D)containing an acid group. The crosslinking agent (G) works to furtherstrengthen the chemical bonds in the crosslinked structure of theorganic/inorganic hybrid, crosslinkable compound (C) and compound (D)containing an acid group and also further enhance extent of crosslinkingthereby contributing to improved membrane properties, e.g., toughnessand gas barrier characteristics.

[0280] A crosslinkable, metallic compound which gives another metaloxide (e.g., that of titanium, zirconium or aluminum oxide) can besuitably used as the crosslinking agent (G). These metallic compoundsinclude mono-, di-, tri- or tetra-alkoxide of titanium, zirconium oraluminum. A crosslinkable metal containing a substituent, e.g., complexwith acetylacetone, may be also incorporated to adjust reactivity. Thecrosslinked structure may be also incorporated with phosphoric,phosphorous or boric acid in combination with the above. Content of thehydrolyzable metallic compound other than silicon compound is notlimited, but preferably 50% by mol or less on the hydrolyzable silylgroup for cost and easiness of controlling the reaction.

[0281] More specifically, the compounds useful for the crosslinkingagent (G) include alkoxysilicates, e.g., tetraethoxysilane,tetramethoxysilane, tetraisopropoxysilane, tetra-n-butoxysilane,tetra-t-butoxysilane, and monoalkyl and dialkyl derivatives thereof,alkoxy titanates, e.g., phenyltriethoxy titanate, halogenated titanates,tetraethoxy titanium, tetra-isopropoxytitanium, tetra-n-butoxytitanium,tetra-t-butoxytitanium, and monoalkyl and dialkyl derivatives thereof,and also oligomers thereof, hydrolyzable zirconium compounds, e.g.,zirconium tetra-n-butoxide, zirconium tetra-t-butoxide, zirconiumtetra-n-propoxide, zirconium tetra-i-propoxide, zirconium tetraethoxide,zirconium tetra(2-methyl-2-butoxide) and zirconiumtetra(2-ethylhexyoxide); hydrolyzable aluminum compounds, e.g.,aluminum-tri-s-butoxide, aluminum-tri-n-butoxide,aluminum-tri-t-butoxide, aluminum-tri-i-propoxide andaluminum-triphenoxide; and phosphoric, phosphorous and boric acid, andesters thereof. The hydrolyzable metallic compound may be formed into acomplex with a β-diketone (e.g., acetylacetone or acetoacetic acidester), ethylene glycol, ethylene glycol (mono- or di-)alkyl ester, orethanol amine, to control the reactivity.

[0282] A hydrolyzable alkoxysilane polymer represented by the generalformula (21) may be also used as the crosslinking agent (G):

[0283] (wherein, R⁶ is CH₃ or C₂H₅ group; and “m” is an integer of 1 to300).

[0284] The hydrolyzable alkoxysilane polymer can be suitably used,because it has adequate crosslinking performance and its reactivityitself is easily controlled. When the organic/inorganic hybrid structure(A) has one silanol group at the terminal, in particular, use of thecrosslinking agent (G) imparts adequate softness and strengthsimultaneously to the membrane.

[0285] The first method of the present invention may use an adequatesolvent to prepare a mixture of an organic/inorganic hybrid,crosslinkable compound (C), compound (D) containing an acid group andcrosslinking agent (G) in the first step. The solvents useful for thepresent invention generally include, but not limited to, alcohols, e.g.,methanol, ethanol, isopropanol, n-butanol and t-butanol; ethers, e.g.,tetrahydrofuran and dioxane; glycol alkyl ethers, e.g., ethylene glycolmonomethyl ether, ethylene glycol monoethyl ether and ethylene glycolmonobutyl ether; and water. Any solvent may be used so long as it isuseful for dissolution or mixing an organic compound, metallic alkoxideor the like.

[0286] Ratio of the solvent is not limited, but the content ispreferably adjusted to give a solids concentration of 80 to 10% byweight.

[0287] The first step for mixing the starting compounds for theprecursors may use another optional component, e.g., reinforcing agent,softening agent, surfactant, dispersant, reaction promoter, stabilizer,colorant, antioxidant, or inorganic or organic filler, within limits notharmful to the object of the present invention.

[0288] The mixing may be achieved by a known method, e.g., agitation orvibration. The mixing method is not limited, so long as it cansufficiently mix the materials. Heating, pressurizing, defoaming,degassing or the like may be used, as required.

[0289] In the first method, the second step follows the first step tomake a thin film of the precursor mixture solution, obtained in thefirst step, by a known method, e.g., casting or coating.

[0290] The film-making method is not limited, so long as it can give auniform film. Film thickness can be optionally varied in a range from 10μm to 1 mm. It is adequately selected in consideration of protonconductivity, fuel permeability and mechanical strength of the film,among others. It is not limited, but preferably, in general, in a rangefrom 30 to 300 μm on a dry basis.

[0291] The film may be incorporated with a support or reinforcing agentof fibers, mat or fibrils during the film-making step. These arepreferably of glass, silicon resin, fluorine resin or polymeric materialin consideration of their resistance to heat and acid.

[0292] In the first method, then, the third step follows the second stepto hydrolyze and condense the crosslinking group (more specifically, thehydrolysable silyl group) in the film prepared in the second step(hydrolysis is saved when the crosslinking group is silanol orsilanolate) to form a crosslinked structure composed of thesilicon-oxygen bond. The crosslinking is achieved by the so-calledsol-gel reaction.

[0293] The objective membrane is produced under heating at an optionaltemperature in a range from room temperature to around 300° C. in thecrosslinking step. It is heated by a known method, e.g., heating in anoven, or in an autoclave under pressure.

[0294] The precursor mixture solution may be incorporated with water orheated in a hydrothermal condition beforehand to perform thehydrolysis/condensation more efficiently in the third step.

[0295] An acid, e.g., hydrochloric, sulfuric or phosphoric acid, may beincorporated as a catalyst beforehand in the reaction system, toaccelerate formation of the crosslinked structure. Formation of thecrosslinked structure can be accelerated also in the presence of a base.Therefore, a basic catalyst, e.g., ammonia or sodium hydroxide, may bealso used.

[0296] It is preferable for the first method to crosslink thehydrolysable silyl group at 100 to 300° C. in the third step, or includean aging step effected at 100 to 300° C. subsequently to the third step.

[0297] When the proton-conducting membrane of the present invention isto be used at a high temperature of 100° C. or higher, it is preferablyheated at service temperature or higher. This can be achieved directlyin the crosslinking step effected at 100 to 300° C., or by heating at100 to 300° C. subsequent to the crosslinking step where the membrane ishardened by the sol-gel process at, e.g., 5 to 40° C. for 2 hours ormore. It can be heated by an ordinary heat source, far-infrared ray,electromagnetic wave induction, microwaves, or a combination thereof.

[0298] The membrane undergoing these steps may be washed with water, asrequired. The washing medium is preferably distilled or ion-exchangedwater free of metallic ion. It may be also treated with sulfuric acid orhydrogen peroxide, to remove impurities and unnecessary metallic ions,and thereby to further increase proton content in the membrane.

[0299] The treated membrane may be further irradiated with ultravioletor electron beams to further increase extent of crosslinking.

[0300] 7.2 Second Method

[0301] This method comprises, as described above, the first step ofpreparing a mixture containing an organic/inorganic hybrid,crosslinkable compound (C) and compound (E), the former having 2 or morecrosslinkable silyl groups and carbon atoms each being bonded to thesilyl group via the covalent bond and the latter having a crosslinkablesilyl group and mercapto group, second step of forming the above mixtureinto a film, the third step of hydrolyzing/condensing the hydrolyzablesilyl group contained in the mixture formed into the film to form acrosslinked structure, and the fourth step of oxidizing the mercaptogroup in the crosslinked structure obtained in the third step intosulfonic acid.

[0302] It is described in more detail for the starting materials andtreatment conditions, among others, for these steps orderly.

[0303] The second method of the present invention first prepares amixture containing an organic/inorganic hybrid, crosslinkable compound(C) and mercapto-containing compound (E) containing a crosslinkablesilyl group and-mercapto group.

[0304] The proton conducting membrane of the present invention shouldsatisfy, as mentioned earlier, the requirements of simultaneouslycontaining an organic/inorganic hybrid structure (A) and anacid-containing structure (B). Therefore, the starting materials eachcorresponding to the organic/inorganic hybrid structure (A) oracid-containing structure (B) are used to form the crosslinkedstructure.

[0305] Accordingly, an organic/inorganic hybrid, crosslinkable compound(C) as the starting material for the organic/inorganic hybrid structure(A) is the same as that for the first method.

[0306] For the starting material for the acid-containing structure (B),on the other hand, a compound (E) containing a mercapto group is used inplace of a compound (D) containing an acid group for the first method. Amercapto group can be converted into sulfonic acid as a functionalgroup, and the acid-containing structure (B) can be obtained byincorporation of the mercapto group followed by its oxidation.

[0307] The compound (E) containing a mercapto group is not limited solong as it has a mercapto group and crosslinkable silyl group. However,a compound represented by the general formula (14) is suitably used:

[0308] (wherein, R⁷ is a molecular chain group having at least onemercapto group; R⁴ is methyl, ethyl, propyl or phenyl group; Rr is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0309] Of these compounds, the compound (E) containing a mercapto groupis more preferably represented by the general formula (15) inconsideration of stability of R⁷ to heat, acid and oxidation:

[0310] (wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is 0, 1 or 2; and “n” isan integer of 1 to 20).

[0311] The compounds represented by the above formula are commerciallyavailable. The known ones include 3-mercaptopropyltrimethoxysilane,3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane,mercaptomethyltrimethoxysilane and mercaptomethyldiethoxysilane.

[0312] The compounds useful for the compound (E) containing a mercaptogroup, other than those commercially available, can be easilysynthesized by, e.g., reaction of sodium hydrosulfide with a halidehaving an alkoxysilyl group and halogenated alkyl, e.g.,3-chloropropyltrimethoxysilane, or hydrolysis of an alkyl group havingan alkoxysilyl group and unsaturated bond, e.g., allyltriethoxysilane,after thioacetic acid or the like is added.

[0313] They may be a polymer or oligomer, which can be synthesized fromthe compound containing a mercapto group as the starting compound.

[0314] Examples of the polymers or oligomers have a structurerepresented by the general formula (16):

[0315] (wherein, R⁶ is H, or CH₃, C₂H₅, C₃H₇ or C₆H₅ group; R⁴ is CH₃,C₂H₅, C₃H₇, C₆H₅, OH, OCH₃, OC₂H₅, OC₆H₅ group, or O—Si bond; “m” is aninteger of 1 to 20; and “n” is an integer of 3 to 500).

[0316] The compound represented by the general formula (16) with R⁴ of ahydrocarbon group is preferable, because of its flexible bond toincrease flexibility of the final membrane. The compound represented bythe general formula (16) with R⁴ of OH or an alkoxy group is alsopreferable, because it can be involved in the crosslinking reaction tomake the bond faster, thereby contributing to improved properties of themembrane, e.g., stabilized conducting characteristics and increasedtoughness. It is more preferable that the compound has a hydrocarbongroup and OH or alkoxy group simultaneously as R⁴ in the same molecule,because flexibility and toughness of the membrane can be controlled inthis case.

[0317] Moreover, “n” in the general formula (16) is preferably 3 ormore, because the oxidation-produced sulfonic acid group has a chainstructure, which accelerates the proton conduction and, at the sametime, provides a strong polar field to allow water to be collected moreefficiently. On the other hand, “n” above 500 is not preferable, becausethe molecule is difficult to synthesize, has reduced compatibility withthe organic/inorganic hybrid, crosslinkable compound (C), and isdifficult to form a uniform membrane. In particular, when R⁴ is ahydrocarbon group, inclusion of the molecule in the membrane isprevented because of reduced number of the functional groups, andsubstantially “n” is preferably 300 or less.

[0318] The compound represented by the general formula (16) with R⁴ ofOCH₃ group, “m” of 3 and “n” of 10 is commercialized by Shin-etsuSilicones (X-41-1805).

[0319] This oligomer can be also easily synthesized, e.g., by the methoddisclosed by Journal of Polymer Science, Part A, Polymer Chemistry,Vol.37, P.1017 (1999). This method can synthesize the oligomer andpolymer having a molecular weight of up to several tens of thousandsusing the mercapto compound represented by the general formula (14) or(15) as the starting material by selecting an adequate reactioncatalyst, and controlling quantity of water for hydrolysis and reactiontime, among others. These oligomers and polymers can be readilyavailable, and can be suitably used for the present invention.

[0320] For example, the compound represented by the general formula (16)with R⁴ of CH₃ group, “m” of 3 and “n” of 3 to 300 can be synthesized bythe method similar to that described in the above literature using3-mercaptopropylmethyldimethoxysilane as a commercial product. These cangive the membrane of high flexibility, and suitably used for the presentinvention.

[0321] Moreover, the oligomer or polymer represented by the generalformula (17) can be also suitably used:

[0322] (wherein, R⁶ is H, or CH₃, C₂H₅, C₃H₇ or C₆H₅ group; R⁴ is CH₃,C₂H₅, C₃H₇, C₆H₅, OH, OCH₃, OC₂H₅ or OC₆H₅ group; R¹¹ is a substitute of6 carbon atoms or less; “m” is an integer of 1 to 20; “n” is an integerof 3 to 500; and “n+x” is an integer of 500 or less, where the unitcontaining mercapto group and that containing R¹¹ may be present in ablock or random form).

[0323] The compound represented by the general formula (17) with R⁴ ofOCH₃ group, “m” of 3 and “n+x” of around 10 is commercialized byShin-etsu Silicones (X-41-1810). These compounds can be easilysynthesized by referring to the above-described literature using 2 ormore alkoxysilane compounds, and can be suitably used for the presentinvention.

[0324] The second method of the present invention makes a thin film inthe second step using the mixture prepared in the first step, and formsa crosslinked structure in the film in the third step. These second andthird steps are similar to those for the first method.

[0325] In addition, a crosslinking agent (G), another additive, solventor the like may be used, as required, as is the case with the firstmethod.

[0326] In the second method of the present invention, the third step isfollowed by the fourth step, where the mercapto group in the crosslinkedstructure obtained in the third step is oxidized into sulfonic acid. Inthis step, the film-like object containing the compound (E) has themercapto group oxidized into sulfonic acid group to become aproton-conducting membrane.

[0327] A common oxidizing agent is used for oxidizing the mercaptogroup. These agents include nitric acid, hydrogen peroxide, oxygen,organic peroxide (percarboxylic acid), bromine water, hypochlorite,hypobromite, potassium permanganate and chromate.

[0328] The oxidation of mercapto group in the presence of the aboveoxidizing agent is a known process, described in Jikken Kagaku Koza(Maruzen, third edition, P.1775) and literature cited therein. Of theseagents, hydrogen peroxide and organic peroxide (e.g., peracetic andperbenzoic acid) are more suitably used, because they are handledrelatively easily and give a high oxidation yield.

[0329] The oxidation-treated membrane may be treated with a strong acid,e.g., hydrochloric or sulfuric acid, to make the sulfonic acid grouptherein protonic. The treatment conditions, e.g., acid concentration,and immersing time and temperature, are adequately determined inconsideration of concentration of the compound containing sulfonic acidgroup in the membrane, membrane porosity and its affinity for the acid.The representative treatment conditions are 1N sulfuric acid in whichthe membrane is immersed at 50° C. for 1 hour.

[0330] 7.3 Third Method

[0331] This method comprises, as described above, the first step ofpreparing a mixture containing an organic/inorganic hybrid,crosslinkable compound (C) and compound (F), the former having 2 or morecrosslinkable silyl groups and carbon atoms each being bonded to thesilyl group via the covalent bond and the latter having a crosslinkablesilyl group and polysulfide group, second step of forming the abovemixture into a film, the third step of hydrolyzing/condensing thehydrolyzable silyl group contained in the mixture formed into the filmto form a crosslinked structure, and the fourth step of oxidizing thepolysulfide group in the crosslinked structure obtained in the thirdstep into sulfonic acid.

[0332] It is described in more detail for the starting materials andtreatment conditions, among others, for these steps orderly.

[0333] The third method of the present invention first prepares amixture containing an organic/inorganic hybrid, crosslinkable compound(C) and compound (F) containing a crosslinkable silyl group andpolysulfide group.

[0334] The proton conducting membrane of the present invention shouldsatisfy, as mentioned earlier, the requirements of simultaneouslycontaining an organic/inorganic hybrid structure (A) and acid-containingstructure (B). Therefore, the starting materials each corresponding tothe organic/inorganic hybrid structure (A) or acid-containing structure(B) are used to form the crosslinked structure.

[0335] Accordingly, an organic/inorganic hybrid, crosslinkable compound(C) as the starting material for the organic/inorganic hybrid structure(A) is the same as that for the first and second method.

[0336] For the starting material for the acid-containing structure (B),on the other hand, a compound (F) containing a polysulfide group is usedin place of a compound (D) containing an acid group for the first methodor compound (E) containing a mercapto group for the second method.

[0337] A polysulfide group can be converted into sulfonic acid as afunctional group by oxidation. The acid-containing structure (B) can beproduced by oxidation of the polysulfide group introduced.

[0338] The polysulfide group means a structure in which sulfur atoms arebonded to each other to form a chain, from a disulfide group (—S—S—)having 2 sulfur atoms to octasulfide group having 8 sulfur atoms.

[0339] Each of these groups is useful for the present invention.However, a disulfide (—S—S—) and tetrasulfide (—S—S—S—S—) group are morepreferable for their general availability.

[0340] The compound (F) containing a polysulfide group is not limited solong as it has a polysulfide group and crosslinkable silyl group.However, a compound represented by the general formula (18) is suitablyused:

[0341] (wherein, R⁸ is a molecular chain group having at least onepolysulfide group; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ isCl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0342] Of these compounds, the compound (F) containing a polysulfidegroup is more preferably represented by the general formula (19), whichhas a structure similar to that of the organic/inorganic hybrid,crosslinkable compound (C), in consideration of reactivity with thecompound (C):

[0343] (wherein, R⁹ is a molecular chain group having at least onepolysulfide group; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ isCl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0344] Of these compound, the compound (F) containing a polysulfidegroup is more preferably represented by the general formula (20) inconsideration of stability of R⁸ to heat, acid and oxidation:

[0345] (wherein, R¹⁰ is a polysulfide group; R⁴ is methyl, ethyl, propylor phenyl group; R⁶ is Cl, or OCH₃, OC₂H₅, OC₆H₅,OH or OCOCH₃ group; “m”is 0, 1 or 2; and “n” is an integer of 1 to 6).

[0346] The compounds represented by the above formula are commerciallyavailable. The known ones includebis[3-(triethoxysilyl)propyl]tetrasulfide (Gelest, Inc. SIB 1825.0), andbis[3-(triethoxysilyl)propyl]disulfide (Shin-etsu Silicones KBE886B).

[0347] The compounds useful for the compound (F), other than thosecommercially available, can be easily synthesized by, e.g., dimerizationof a thiol or ionic or radical addition of sulfur (S₈), and hencesynthesized from a precursor which simultaneously has an inorganic,crosslinkable group and a group reactive with the sulfur compounddescribed above.

[0348] The third method of the present invention makes a thin film inthe second step using the mixture prepared in the first step, and formsa crosslinked structure in the film in the third step. These second andthird steps are similar to those for the first and second methods.

[0349] In addition, a crosslinking agent (G), another additive, solventor the like may be used, as required, as is the case with the first andsecond methods.

[0350] In the third method of the present invention, the third step isfollowed by the fourth step, where the polysulfide group in thecrosslinked structure obtained in the third step is oxidized intosulfonic acid. In this step, the film-like object containing thecompound (F) has the polysulfide group oxidized into sulfonic acid groupto become a proton-conducting membrane.

[0351] The fourth step for oxidizing a polysulfide group in the thirdmethod can be similar to that of the fourth step for oxidizing amercapto group in the second method.

[0352] Oxidation of a mercapto group differs from that of a polysulfidegroup in that the former gives one sulfonic acid group per one mercaptogroup whereas the latter gives 2 sulfonic acid groups per onepolysulfide group, when a polysulfide-containing compound has astructure represented by the general formula (19) or (20).

[0353] Therefore, when a polysulfide-containing compound (F) having astructure represented by the general formula (19) or (20) is used, aproton-conducting membrane can be obtained at a mixing ratio of anorganic/inorganic hybrid, crosslinkable compound (C) to thepolysulfide-containing compound (F) from 95:5. The membrane has asufficient strength when the ratio is kept at 10:90 or higher. Themixing ratio is preferably in the above range, accordingly.

[0354] 7.4 Fourth Method

[0355] This method comprises, as described above, the first step ofpreparing a mixture containing an organic/inorganic hybrid,crosslinkable compound (C) and compound (H), the former having 2 or morecrosslinkable silyl groups and carbon atoms each being bonded to thesilyl group via the covalent bond and the latter having a crosslinkablesilyl group and halogen group, the second step of forming the abovemixture into a film, the third step of hydrolyzing/condensing thehydrolyzable silyl group contained in the mixture formed into the filmto form a crosslinked structure, and the fourth step of substituting thehalogen group in the crosslinked structure obtained in the third stepwith sulfonic acid group.

[0356] It is described in more detail for the starting materials andtreatment conditions, among others, for these steps orderly.

[0357] The fourth method of the present invention first prepares amixture containing an organic/inorganic hybrid, crosslinkable compound(C) and compound (H) containing a crosslinkable silyl group and halogengroup.

[0358] The proton conducting membrane of the present invention shouldsatisfy, as mentioned earlier, the requirements of simultaneouslycontaining an organic/inorganic hybrid structure (A) and acid-containingstructure (B). Therefore, the starting materials each corresponding tothe organic/inorganic hybrid structure (A) or acid-containing structure(B) are used to form the crosslinked structure.

[0359] Accordingly, an organic/inorganic hybrid, crosslinkable compound(C) as the starting material for the organic/inorganic hybrid structure(A) is the same as that for the first to the third methods.

[0360] For the starting material for the acid-containing structure (B),on the other hand, a compound (H) containing a halogen group is used inplace of a compound (D) containing an acid group for the first method,compound (E) containing a mercapto group for the second method, orcompound (F) containing a polysulfide group for the third method.

[0361] The halogen group means chlorine, bromine, iodine or the like. Inaddition to a halogen group, a leaving group may be also used so long asit is reactive with sulfurous acid for substitution. These groupsinclude tosylate and methanesulfonylate. However, the halogen group ismore preferable than the leaving group, because it is generally moreeasily available.

[0362] A halogen group can be converted into sulfonic acid as afunctional group by substitution with sulfurous acid. The reaction isknown, and disclosed by, e.g., Organic Syntheses Collective Volume 2(1943), P.558 and 564. The substitution reaction can be carried outeasily and in a high yield by heating, e.g., sodium sulfite and acompound containing a halogen group in a mixed solvent of alcohol andwater. The reaction product is generally in the form of salt, and may bepost-treated with sulfuric acid or the like to be protonic.

[0363] The compound (H) containing a halogen group is not limited solong as it has a halogen group and crosslinkable silyl group. However, acompound represented by the general formula (22) is suitably used:

[0364] (wherein, R¹² is a molecular chain group having at least onehalogen group; R⁴ is methyl, ethyl, propyl or phenyl group; Rr, is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).

[0365] Of these compounds, the compound (H) containing a halogen groupis more preferably represented by the general formula (23) inconsideration of stability of R¹² to heat, acid and oxidation:

[0366] (wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl,or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; X is Cl, Br or I; “n” is aninteger of 1 to 20; and “m” is 0, 1 or 2).

[0367] The compounds represented by the above formula are commerciallyavailable. The known ones include chloromethyldimethylchlorosilane,chloromethyldimethylethoxysilane, chloromethyldimethylisopropoxysilane,chloromethylmethyldiethoxysilane, chloromethymethyldiisopropoxysilane,chloromethyltrichlorosilane, chloromethyltriethoxysilane,chloromethyltrimethoxysilane, 2-chloroethylmethyldichlorosilane,2-chloroethylmethyldimethoxysilane, 2-chloroethyltrichlorosilane,2-chloroethyltriethoxysilane, 2-chloroethyltrimethoxysilane,3-chloropropyldimethylchlorosilane, 3-chloropropyldimethylmethoxysilane,3-chloropropylmethyldichlorosilane, 3-chloropropylmethyldimethoxysilane,3-chloropropylphenyldichlorosilane, 3-chloropropyltilichlorosilane,3-chloropropyltriethoxysilane, 3-chloropropyltrimethoxysilane,4-chlorobutyldimethylchlorosilane, bromomethyldimethylchlorosilane,2-bromoethyltrichlorosilane, 3-bromopropyltrichlorosilane,3-bromopropyltriethoxysilane, 3-bromopropyltrimethoxysilane,11-bromoundecyldimethylchlorosilane, 11-bromoundecyltrichlorosilane,11-bromoundecyltrimethoxysilane and 3-iodopropyltrimethoxysilane. Thesecompounds are readily available, although the compounds useful for thepresent invention are not limited to the above.

[0368] These compounds can be easily synthesized by reacting a compoundhaving a halogen group at one terminal and unsaturated bond at the otherterminal, represented by the general formula (24), with trialkoxysilane,dialkoxyalkylsilane, monoalkoxydialkylsilane or the like forhydrosilylation:

[0369] (wherein, “n” is an integer of 1 to 18).

[0370] For the hydrosilylation, bromine or iodine as the halogen groupis more preferable than chlorine.

[0371] These compounds may be used as the oligomers or polymers,synthesized by the method disclosed by, e.g., Journal of PolymerScience, Part A, Polymer Chemistry, Vol.37, P.1017 (1999)), citedearlier.

[0372] The fourth method of the present invention makes a thin film inthe second step using the mixture prepared in the first step, and formsa crosslinked structure in the film in the third step. These second andthird steps are similar to those for the first to third methods.

[0373] In addition, a crosslinking agent (G), another additive, solventor the like may be used, as required, as is the case with the first tothird methods.

[0374] In the fourth method of the present invention, the third step isfollowed by the fourth step, where the halogen group in the crosslinkedstructure obtained in the third step is converted into sulfonic acid orits salt by substitution. In this step, the film-like object containingthe compound (H) has a halogen group converted into sulfonic acid or itssalt to become a proton-conducting membrane.

[0375] In this substitution process, the film-like object containing thehalogen-containing compound (H) is reacted with a sulfite compound,e.g., sulfurous acid (aqueous solution), sodium sulfite, potassiumsulfite, sodium hydrogen sulfite or potassium hydrogen sulfite, forwhich the method disclosed by Organic Syntheses Collective Volume 2(1943), P.558 and 564 may be used. More specifically, a simple methodinvolves heating the crosslinked membrane produced in an alcoholicaqueous solution containing sulfite ion.

[0376] The halogen group can be converted into sulfonic acid or its saltin the above step. When the salt is produced, the membrane is immersedin sulfuric acid or the like, to become a proton-conducting membrane.

[0377] When a halogen-containing compound (H) having a structurerepresented by the general formula (22) or (23) is used, aproton-conducting membrane can be obtained at a mixing ratio of anorganic/inorganic hybrid, crosslinkable compound (C) to thehalogen-containing compound (H)Y from 1:9. The membrane has a sufficientstrength when the ratio is kept at 9:1 or lower. The mixing ratio ispreferably in the above range, accordingly.

[0378] 7.5 Method for Compositing a Fibrous Material (I)

[0379] As described earlier, a composite membrane containing a fibrousmaterial (I) is one of the preferred embodiments of theproton-conducting membrane of the present invention. One of the first tofourth methods is basically applicable to production of such a compositemembrane.

[0380] More specifically, each of the above methods further comprises anadditional step for compositing the fibrous material. Some of theexamples of this step are described below, which by no means limit thepresent invention. This step varies depending on, e.g., type andquantity of the fibrous material, as described below.

[0381] When the fibrous material (I) is a short fibrous material (J),e.g., short glass fibers (M) or whiskers (L), it may be added to themixture in the first step of one of the first to fourth methods. It ispreferably mixed with the mixture by a homogenizer or ball mill, whichinvolves strong agitation to prevent agglomeration. The mixture ispreferably used immediately after it is prepared to prevent separation,or else treated to have a certain viscosity to prevent the mixture frombeing easily separated.

[0382] When excessively incorporated, the short fibrous material (J) maynot be dispersed sufficiently to possibly cause excessive permeation ofthe gas, and may decrease conductivity of the membrane. Whenincorporated insufficiently, it may not fully exhibit its effect.Therefore, it is incorporated at 1 to 75 parts by weight per 100 partsby weight of the organic/inorganic hybrid structure (A) andacid-containing structure (B) totaled.

[0383] On the other hand, a long fibrous material (K) as the fibrousmaterial (I) cannot directly form a uniform composite structure, evenwhen mixed beforehand with the starting materials for theorganic/inorganic hybrid structure (A) and acid-containing structure(B), unlike the short fibrous material (J). Therefore, it preferably hasa specific shape (sheet shape) before it is incorporated.

[0384] The long fibrous material (K), to be incorporated in the secondstep of one of the first to fourth methods, is preferably loaded with astarting mixture of the organic/inorganic hybrid structure (A) andacid-containing structure (B). It may be loaded by pressing or theliquid, starting mixture on the sheet, or rolling. The loading methodcan be optionally selected from the known, simple ones.

EXAMPLES AND COMPARATIVE EXAMPLES

[0385] The present invention is described by EXAMPLES, which by no meanslimit the present invention. All of the compounds, solvents and the likeused in EXAMPLES and COMPARATIVE EXAMPLES were commercial ones. Theywere used directly, i.e., not treated for these examples. Properties ofthe proton conducting membrane prepared were evaluated by the analyticalmethods described below.

[0386] Analytical Methods

[0387] (1) Evaluation of Membrane Properties

[0388] The proton conducting membrane was subjected to the bendingfunctional test, and its properties were rated according to thefollowing standards:

[0389] ∘: The membrane can be bent, and is kept flexible.

[0390] x: The membrane cannot be bent.

[0391] (2) Evaluation of Proton Conductivity at Low Temperature

[0392] The proton conducting membrane of the present invention wascoated with carbon paste (Conducting Graphite Paint: LADO RESEARCHINDUSTRIES, INC.) on both sides, to which platinum plates were fastadhered. It was analyzed for its impedance by an electrochemicalimpedance meter (Solartron 1260) in a frequency range from 0.1 Hz to 100kHz, to determine its proton conductivity.

[0393] In the above analysis, the sample was supported in anelectrically insulated closed container, and measured for its protonconductivity at varying temperature in a water vapor atmosphere (95 to100% RH), where cell temperature was increased from room temperature to160° C. by a temperature controller. Proton conductivity was measured ateach temperature level, and the value measured at 60° C. is reported inthis specification as the representative one. Moreover, the resultsobtained only at 140° C., or 60° C. and 160° C. are also reported forrepresentative EXAMPLES. For the measurement at 100° C., the measurementtank was pressurized.

[0394] (3) Evaluation of Heat Resistance

[0395] The proton conducting membrane was heated at 140° C. for 5 hoursin an autoclave in a saturated steam atmosphere. The treated membranewas evaluated for its heat resistance by the visual and bendingfunctional tests, and its heat resistance was rated according to thefollowing standards:

[0396] ∘: No change is observed before and after the treatment.

[0397] x: Embrittlement, disintegration, discoloration or deformation ofthe treated membrane is observed.

Example 1

[0398] A solution of 0.9 g of 1,8-bis(triethoxysilyl)octane (Gelest,Inc.) dissolved in 1.5 g of isopropyl alcohol was prepared. Anothersolution of 1.5 g of isopropanol added to 1.8 g of a 33% aqueoussolution of 3-(trihydroxysilyl)propanesulfonic acid was separatelyprepared. These solutions were mixed with each other, stirred forseveral minutes, and poured into a Petri dish of polystyrene (YamamotoSeisakusho, inner diameter: 8.4 cm), where the mixture was left at roomtemperature (20° C.) for 15 hours, and heated at 80° C. for 10 hours ina saturated steam atmosphere and at 100° C. in an oven, to prepare thetransparent, flexible membrane. It was washed in a flow of water at 60°C. for 2 hours, before it was analyzed. The evaluation results and thelike of the membrane are given in Table 1.

Example 2

[0399] A membrane was prepared in the same manner as in EXAMPLE 1,except that 1.3 g of 1,8-bis(triethoxysilyl)octane (Gelest, Inc.) and0.8 g of a 33% aqueous solution of 3-(trihydroxysilyl)propanesulfonicacid were used. The transparent, tough membrane was obtained. Theevaluation results and the like of the membrane are given in Table 1.

Example 3

[0400] A membrane was prepared in the same manner as in EXAMPLE 1,except that 0.7 g of 1,8-bis(triethoxysilyl)octane (Gelest, Inc.) and2.3 g of a 33% aqueous solution of 3-(trihydroxysilyl)propanesulfonicacid were used. The transparent, slightly fragile membrane was obtained.The evaluation results and the like of the membrane are given in Table1.

Example 4

[0401] (Synthesis of Bifunctional Precursor)

[0402] A solution of 11.0 g of 1,7-octadiene (Wako Pure ChemicalIndustries) and 26.9 g of diethoxymethylsilane (Shin-etsu Silicones)dissolved in toluene was incorporated with 0.05 mmols of a solution ofKarstedt catalyst (U.S. Pat. No. 3,775,452) prepared from achloroplatinate (Wako Pure Chemical Industries) and divinyltetramethyldisiloxane (Gelest, Inc.), and the resulting mixture wasstirred at 30° C. in a nitrogen atmosphere for 24 hours. The reactioneffluent was purified by distillation, to obtain1,8-bis(diethoxymethylsilyl)octane. Its structure was confirmed by NMRanalysis.

[0403] (Preparation of Mixture and its Film)

[0404] A membrane was prepared in the same manner as in EXAMPLE 1,except that 1.0 g of 1,8-bis(diethoxymethylsilyl)octane prepared aboveand 1.5 g of a 33% aqueous solution of3-(trihydroxysilyl)propanesulfonic acid were used. The transparent,highly flexible membrane was obtained. The evaluation results and thelike of the membrane are given in Table 1.

Example 5

[0405] A membrane was prepared in the same manner as in EXAMPLE 1,except that 0.9 g of 1,6-bis(trimethoxysilyl)hexane (Gelest, Inc.) and1.7 g of a 33% aqueous solution of 3-(trihydroxysilyl)propanesulfonicacid were used. The transparent, slightly fragile membrane was obtained.The evaluation results are given in Table 1.

Example 6

[0406] (Synthesis of 1,14-bis(triethoxysilyl)tetradecane)

[0407] The synthesis method is described in detail by, e.g., H. W.Oviatt et al., Chem. Mater., 1993, 5, 943, and1,14-bis(triethoxysilyl)tetradecane was synthesized following the abovemethod. A mixture of 25 g of 1,13-tetradecadiene (Aldrich), 44.4 g oftriethoxysilane (Shin-etsu Silicones) and 0.1 mL of a 3% xylene solutionof platinum complex of bis((vinyl dimethyldisiloxane) (Shin-etsuSilicones) was prepared and stirred at room temperature in a nitrogenatmosphere for 3 days. The reaction effluent was purified bydistillation, to obtain 1,14-bis(triethoxysilyl)tetradecane. Itsstructure was confirmed by NMR analysis.

[0408] (Preparation of Mixture and its Film)

[0409] A membrane was prepared in the same manner as in EXAMPLE 1,except that 1.1 g of 1,14-bis(triethoxysilyl)tetradecane prepared aboveand 1.3 g of a 33% aqueous solution of3-(trihydroxysilyl)propanesulfonic acid were used. The transparent,highly flexible membrane was obtained. The evaluation results and thelike of the membrane are given in Table 1.

Example 7

[0410] A membrane was prepared in the same manner as in EXAMPLE 1,except that 1.0 g of a compound having 10 dimethylsiloxane chains bondedto each other in series and trimethoxysilyl groups at both terminals(Shin-etsu Silicones, X-40-2090) and 1.5 g of a 33% aqueous solution of3-(trihydroxysilyl)propanesulfonic acid were used. The whitely turbid,highly flexible membrane was obtained. The evaluation results and thelike of the membrane are given in Table 1.

Example 8

[0411] A membrane was prepared in the same manner as in EXAMPLE 1,except that 0.3 g of 1N hydrochloric acid was added to 0.5 g of3-mercaptopropyltrimethoxysilane (CHISSO CORPORATION, SILA-ACE S810) and1.0 g of bis(diethoxymethylsilyl)octane, and the water washing step wassaved.

[0412] The membrane was immersed in 7.0 mL of glacial acetic acid, towhich 5.6 mL of a 30% aqueous solution of hydrogen peroxide was addedlittle by little at 70° C. Then, the system was heated to 70° C., atwhich it was held for 20 minutes. The membrane was withdrawn from thesystem, after it was cooled, immersed in 1N sulfuric acid over a night,and then washed in a flow of water at 60° C. for 2 hours. The evaluationresults and the like of the membrane are given in Table 1.

Example 9

[0413] A membrane was prepared in the same manner as in EXAMPLE 1,except that 0.5 g of a polydimethylsiloxane compound:(moolecular weight:400 to 700, Gelest, Inc., DMS-S12) having dimethylhydroxysilyl groups atboth terminals, 0.5 g of diethoxysiloxane (Gelest, Inc., PSI-021) as acrosslinking promoter and 1.5 g of a 33% aqueous solution of3-(trihydroxysilyl)propanesulfonic acid were used. The highly flexiblemembrane was obtained. The evaluation results and the like of themembrane are given in Table 1.

Example 10

[0414] A membrane was prepared in the same manner as in EXAMPLE 9,except that the polydimethylsiloxane compound havingdimethylhydroxysilyl groups at both terminals was replaced by 0.5 g of apolydimethylsiloxane-polydiphenylsiloxane copolymer (molecular weight:900 to 1000, Gelest, Inc., PDS-1615) having hydroxysilyl groups at bothterminals. The highly flexible membrane was obtained. The evaluationresults and the like of the membrane are given in Table 1.

Example 11

[0415] A membrane was prepared in the same manner as in EXAMPLE 1,except that 0.1 g of a polydimethylsiloxane compound (molecular weight:400 to 700, Gelest, Inc., DMS-S12) having dimethylhydroxysilyl groups atboth terminals, 0.4 g of 1,8-bis(triethoxysilyl)octane (Gelest, Inc.),0.5 g of diethoxysiloxane (Gelest, Inc., PSI-021) and 1.5 g of a 33%aqueous solution of 3-(trihydroxysilyl)propanesulfonic acid were used.The highly flexible membrane was obtained. The evaluation results andthe like of the membrane are given in Table 1.

Example 12

[0416] A membrane was prepared in the same manner as in EXAMPLE 1,except that 0.7 g of 1,8-bis(triethoxysilyl)octane (Gelest, Inc.), 0.2 gof diethoxysiloxane (Gelest, Inc., PSI-021) and 2.3 g of a 33% aqueoussolution of 3-(trihydroxysilyl)propanesulfonic acid were used. Thehighly flexible membrane was obtained. The evaluation results and thelike of the membrane are given in Table 1.

Example 13

[0417] A membrane was prepared in the same manner as in EXAMPLE 1,except that 0.6 g of bis[3-(triethoxysilyl)propyl]tetrasulfide (Gelest,Inc. SIB1825.0), 1.1 g of bis3-(triethoxysilyl)octane and 0.3 g of 1Nhydrochloric acid were used, and the water washing step was saved. Themembrane was immersed in 20 g of a 30% aqueous solution of hydrogenperoxide at 50° C. continuously for 3 hours.

[0418] The membrane was withdrawn from the system, after it was cooled,immersed in 1N sulfuric acid over a night, and then washed in a flow ofwater at 60° C. for 2 hours. The evaluation results and the like of themembrane are given in Table 1.

Example 14

[0419] A membrane was prepared in the same manner as in EXAMPLE 13,except that 0.6 g of bis3-(triethoxysilylpropyl)disulfide (Shin-etsuSilicones KBE886B) and 1.3 g of bis(triethoxysilyl)octane were used. Theevaluation results and the like of the membrane are given in Table 1.

Example 15

[0420] 1,22-bis(triethoxysilyl)docosane was synthesized using1,21-docosadiene (Fluka) as the starting compound in a manner similar tothat for EXAMPLE 6, and its structure was confirmed by NMR analysis. Amembrane was prepared in the same manner as in EXAMPLE 8, except thatbis(triethoxysilyl)octane was replaced by1,22-bis(triethoxysilyl)docosane prepared above. The evaluation resultsand the like of the membrane are given in Table 1.

Example 16

[0421] Bis(ethoxydimethylsilyl)octane was synthesized using octadieneand ethoxydimethylsilane (Gelest, Inc.) as the starting compounds in amanner similar to that for EXAMPLE 6, and its structure was confirmed byNMR analysis. A membrane was prepared in the same manner as in EXAMPLE8, except that bis(triethoxysily)octane was replaced by an equimolarmixture of bis(diethoxymethylsilyl)octane andbis(ethoxydimethylsilyl)octane prepared above. The evaluation resultsand the like of the membrane are given in Table 1.

Example 17

[0422] A membrane was prepared in the same manner as in EXAMPLE 8,except that 3-mercaptopropyltrimethoxysilane was replaced by3-mercaptopropyltrimethoxysilane oligomer (Shin-etsu SiliconesX-41-1805). The oligomer's molar equivalent was set at the same level asthat of the mercapto group. The evaluation results and the like of themembrane are given in Table 1.

Example 18

[0423] A membrane was prepared in the same manner as in EXAMPLE 8,except that 3-mercaptopropyltrimethoxysilane was replaced by3-mercaptopropyltrimethoxysilane-methyltrimethoxysilane cooligomer(Shin-etsu Silicones X-41-1810). The cooligomer's molar equivalent wasset at the same level as that of the mercapto group. The evaluationresults and the like of the membrane are given in Table 1.

Example 19

[0424] (Synthesis of 3-mercaptopropyltrimethoxysilane Polymer)

[0425] 10 mmols of 3-mercaptopropyltrimethoxysilane (Gelest, Inc.) wasincorporated with 1N hydrochloric acid and 1 mL of ethanol, wherequantity of the hydrochloric acid was set to have 15 mmols of water, andthe mixture was stirred at room temperature for 10 minutes. It was thenstirred on a hot plate kept at 80° C. continuously for 2 hours. Part ofthe resulting viscous liquid was analyzed by GPC (JASCO Corp., Column:TOSOH Corp.) for molecular weight. It had a weight-average molecularweight of around 5,000 as polystyrene. It was diluted with acetone to20% by weight, and filtered by a 0.45 μm PTFE membrane filter (ToyoRoshi), to obtain the polymer.

[0426] (Preparation of the Polymer Film)

[0427] A membrane was prepared in the same manner as in EXAMPLE 17. Theevaluation results and the like of the membrane are given in Table 1.

Example 20

[0428] An oligomer was prepared in the same manner as in EXAMPLE 19,except that 3-mercaptopropylmethyldimethoxysilane (Gelest, Inc.) wasused as the starting compound and mixture was stirred under heating for30 minutes. It had a molecular weight of around 1,300. A membrane wasprepared in the same manner as in EXAMPLE 19, except that the oligomerprepared above was used. The evaluation results and the like of themembrane are given in Table 1.

Example 21

[0429] 1.0 g of a 70% isopropanol solution of zirconiumtetra-1-propoxide (Wako Pure Chemical Industries) was incorporatedbeforehand with 0.5 g of ethylene glycol monoethyl ether to prepare astable complex. Then, 0.7 g of bis(ethoxydimethylsilyl)octane wasincorporated with 0.5 g of the stabilized zirconia complex solution. Themixture was stirred for 10 minutes, and incorporated with 0.2 g ofbis(diethoxymethylsilyl)octane, 0.5 g of3-mercaptopropyltrimethoxysilane and 0.3 g of 1N hydrochloric acid. Itwas made into a membrane in a manner similar to that for EXAMPLE 8. Themembrane whitely turbid to some extent was obtained. The evaluationresults and the like of the membrane are given in Table 1.

Example 22

[0430] A membrane was prepared in the same manner as in EXAMPLE 21,except that a 70% isopropanol solution of zirconium tetra-1-propoxidewas replaced by that of titanium tetra-n-butoxide (Wako Pure ChemicalIndustries). The orange-colored membrane was obtained. The evaluationresults and the like of the membrane are given in Table 1.

Example 23

[0431] A mixture of 11.0 g of 3-bromopropyltrimethoxysilane (Shin-etsusilicones) and 1.5 g of bis(triethoxysilyl)octane was incorporated with0.3 g of 1N hydrochloric acid dissolved in 3.0 g of isopropanol. Theresulting mixture was directly cast into a Petri dish of polystyrene(inner diameter: 9 cm), where it was left at room temperature for 24hours, and heated at 80° C. for 12 hours in a saturated steam atmosphereand at 130° C. in an oven for 5 hours. The resulting membrane was washedin distilled water at room temperature for 1 hour. It was then immersedin a mixed solvent of 10 mL of ethanol and 10 mL of water, to which anaqueous solution of 2.5 g of sodium sulfite dissolved in 10 mL of waterwas added little by little, and the mixture was heated with moderatestirring to 90° C., at which it was held for 2 hours at 90° C. Themembrane was withdrawn from the reaction effluent, washed with watersufficiently and immersed in water kept at 80° C., to obtain themembrane whitely turbid to some extent. It was immersed in 1N sulfuricacid over a night at room temperature. The evaluation results and thelike of the membrane are given in Table 1.

Example 24

[0432] A membrane was prepared in the same manner as in EXAMPLE 1,except that 0.1 g of whiskers as short fibers (diameter: 0.3 to 0.6 μm,average length: 10 to 20 μm, aspect ratio: about 30, material:K₂O.6TiO₂, Otsuka Chemical TISMO) were further incorporated and themixture was stirred by a homogenizer. The white, tough membrane wasobtained. The evaluation results and the like of the membrane are givenin Table 1.

Example 25

[0433] A 50 cm thick thirled, square-weave fabric of glass fibers (NittoBoseki WEA05E) put on a Teflon^(R) sheet was loaded with the samestarting solution for the membrane as that used in EXAMPLE 1 using aroller in place of a Petri dish. Quantity of the solution loaded was setat 50 g/m², and rolling was carried out twice. Then, it was treated inthe same manner as in EXAMPLE 1, to prepare the membrane. It showed nofailure when subjected to the tensile strength test and bending testwhere it was exposed to cyclic load. The evaluation results and the likeof the membrane are given in Table 1.

Example 26

[0434] A starting solution for the membrane was prepared in the samemanner as in EXAMPLE 8, except that 0.5 g of whiskers as short fibers(diameter: 0.5 to 1.0 μm, average length: 10 to 30 cm, aspect ratio:about 20 to 30, material: 9Al₂O₃.2B₂O₃, Shikoku Corp. ALBOREX) werefurther incorporated and the mixture was stirred by a homogenizer. A 30μm thick thirled, square-weave fabric of glass fibers (Nitto BosekiWEA05C) put on a Teflon^(R) sheet was loaded with the above startingsolution using a roller in place of a Petri dish. Quantity of thesolution loaded was set at 50 g/m², and rolling was carried out twice.Then, it was treated in the same manner as in EXAMPLE 8, to prepare themembrane. It showed no failure when subjected to the tensile strengthtest and bending test where it was exposed to cyclic load, and wastough. The evaluation results and the like of the membrane are given inTable 1.

Example 27

[0435] The same 50 μm thick thirled, square-weave fabric of glass fibers(Nitto Boseki WEA05E) as that used for EXAMPLE 25 was immersedbeforehand in an aqueous solution of trihydroxysilylpropane sulfonicacid (Gelest, Inc.) as a silane coupling agent for 1 hour, and theas-treated fabric, i.e., the fabric still containing the aqueoussolution, was heated at 80° C. in an oven for 12 hours. The dried glassfibers were washed with water for 2 hours, to remove the surplus silanecoupling agent. The fiber by itself had a conductivity of 3×10⁻³ S/cm. Amembrane was prepared using the glass fibers treated beproton-conducting in a manner similar to that for example 25. Themembrane showed conditions similar to that prepared in EXAMPLE 25. Theevaluation results and the like of the membrane are given in Table 1.

Example 28

[0436] A porous membrane of fluorine resin (Nihon Millipore membranefilter JG, pore diameter: 0.2 cm, thickness: 60 μm) put on a Teflon^(R)sheet was loaded with the same starting solution for the membrane asthat used in EXAMPLE 17 using a roller in place of a Petri dish.Quantity of the solution loaded was set at 50 g/m². Then, it was treatedin the same manner as in EXAMPLE 17, to prepare the membrane. It showedno failure when subjected to bending or the like, and was good inhandleability. The evaluation results and the like of the membrane aregiven in Table 1.

Example 29

[0437] A single-cell fuel cell was assembled using the membrane preparedin EXAMPLE 4, where the membrane put between gas diffusing electrodes(ETEK, loaded with 2.0 mg of platinum) was assembled in a single cell(Electrochem, membrane area: 5.25 cm²). The fuel cell working withhydrogen and oxygen supplied to the respective anode and cathodeproduced a voltage-current curve shown in FIG. 1, when its output wasconnected to an electronic load. TABLE 1-1 Carbon-containing organicchain in the Organic/inorganic organic/inorganic Type of Type ofcomposite, crosslinkable composite structure Starting compound for thestructure starting acid compound (C) (A) (B) containing an acid groupcompound group Example 1 Bis(triethoxysilyl) Octamethylene3-Trihydroxysilylpropanesulfonic (D) Sulfonic octane acid acid Example 2Bis(triethoxysilyl) Octamethylene 3-Trihydroxysilylpropanesulfonic (D)Sulfonic octane acid acid Example 3 Bis(triethoxysilyl) Octamethylene3-Trihydroxysilylpropanesulfonic (D) Sulfonic octane acid acid Example 4Bis(diethoxymethysilyl) Octamethylene 3-Trihydroxysilylpropanesulfonic(D) Sulfonic octane acid acid Example 5 Bis(triethoxysilyl)Hexamethylene 3-Trihydroxysilylpropanesulfonic (D) Sulfonic hexane acidacid Example 6 Bis(triethoxysilyl) Tetradecanethylene3-Trihydroxysilylpropanesulfonic (D) Sulfonic tetradecane acid acidExample 7 Bis(triethoxysilyl) Dimethylsiloxane3-Trihydroxysilylpropanesulfonic (D) Sulfonic polydimethylsiloxane acidacid Example 8 Bis(diethoxymethylsilyl) Octamethylene3-Mercaptopropyltrimethoxysilane (E) Sulfonic octane acid Example 9Polydimethylsiloxane Dimethylsiloxane 3-Trihydroxysilylpropanesulfonic(D) Sulfonic with OH at the terminal acid acid Example 10Polydimethylsiloxane with Polydimethylsiloxane-3-Trihydroxysilylpropanesulfonic (D) Sulfonic OH at the terminal-polydiphenylsiloxane acid acid polydiphenylsiloxane copolymer copolymer(C)/(8) Evaluation molar ratio Evaluation (3) in the (1) Evaluation (2)Evaluation (2) Heat starting Bending Conductivity at Conductivity atresistance at mixture test 60° C. (s/cm) 140° C. (s/cm) 140° C. RemarksExample 1 5:5 ◯ 4.6 × 10⁻² 3.0 × 10⁻² ◯ Example 2 7:3 ◯ 1.0 × 10⁻² 8.9 ×10⁻¹ ◯ Tough Example 3 4:6 ◯ 7.5 × 10⁻² 5.5 × 10⁻² ◯ Slightly fragileExample 4 5:5 ◯ 2.2 × 10⁻² 1.8 × 10⁻² ◯ Highly flexible Example 5 5:5 ◯5.0 × 10⁻² 3.2 × 10⁻² ◯ Slightly fragile Example 6 5:5 ◯ 9.2 × 10⁻³ Notmeasured ◯ Example 7 Details not ◯ 8.7 × 10⁻³ Not measured ◯ Highlyflexible, known Whitely turbid Example 8 3:7 ◯ 5.5 × 10⁻² 6.5 × 10⁻² ◯Mercapto group oxidized with peracetic acid Example 9 3:7 ◯ 4.0 × 10⁻²3.5 × 10⁻² ◯ Flexible Example 10 1.5:8.5 ◯ 3.8 × 10⁻² 3.6 × 10⁻² ◯Flexible

[0438] TABLE 1-2 Carbon-containing organic chain in theOrganic/inorganic organic/inorganic Type of Type of composite,crosslinkable composite structure Starting compound for the structurestarting acid compound (C) (A) (B) containing an acid group compoundgroup Example 11 Bis(triethoxysilyl)octane, Octamethylene,3-Trihydroxysilylpropanesulfonic (D) Sulfonic PolydimethylsiloxaneDimethylsiloxane acid acid with OH at the terminal Example 12Bis(triethoxysilyl) Octamethylene 3-Trihydroxysilylpropanesulfonic (D)Sulfonic octane acid acid Example 13 Bis(triethoxysilyl) OctamethyleneBis[3 (triethoxysilyl)propyl] (F) Sulfonic octane tetrasulfide acidExample 14 Bis(triethoxysilyl) Octamethylene Bis[3(triethoxysilyl)propyl] (F) Sulfonic octane disulfide acid Example 15Bis(triethoxysilyl) Docosanethylene 3-Mercaptopropyltrimethoxysilane (E)Sulfonic docosane acid Example 16 1:1 Mixture of Octamethylene3-Mercaptopropyltrimethoxysilane (E) Sulfonic bis(diethoxymethylsilyl)acid octane + bis(ethoxydimethylsilyl) octane Example 17Bis(diethoxymethylsilyl) Octamethylene 3-Mercaptopropyltrimethoxysilane(E) Sulfonic octane oligomer acid Example 18 Bis(diethoxymethylsilyl)Octamethylene 3-Mercaptopropyltrimethoxysilane (E) Sulfonic octaneoligomer acid Example 19 Bis(diethoxymethylsilyl) Octamethylene3-Mercaptopropyltrimethoxysilane (E) Sulfonic octane polymer acidExample 20 Bis(diethoxymethylsilyl) Octamethylene 3- (E) Sulfonic octaneMercaptopropyldimethoxymethylsilane acid oligomer (C)/(8) Evaluationmolar ratio Evaluation (3) in the (1) Evaluation (2) Evaluation (2) Heatstarting Bending Conductivity at Conductivity at resistance at mixturetest 60° C. (s/cm) 140° C. (s/cm) 140° C. Remarks Example 11 3:7 ◯ 1.1 ×10⁻¹ 9.6 × 10⁻² ◯ Crosslinking agent (G) incorporated Example 12 3:7 ◯7.8 × 10⁻² 8.2 × 10⁻² ◯ Crosslinking agent (G) incorporated Example 137:3 ◯ 3.2 × 10⁻² 3.5 × 10⁻² ◯ Tetrasulfide group oxidized with H2O2Example 14 7:3 ◯ 4.5 × 10⁻² 4.3 × 10⁻² ◯ Disulfide group oxidized withperacetic acid Example 15 3:7 ◯ 4.2 × 10⁻² 3.5 × 10⁻¹ ◯ Highly flexibleExample 16 3:7 ◯ 3.9 × 10⁻² 3.2 × 10⁻² ◯ Highly flexible Example 17 3:7◯ 1.2 × 10⁻¹ 9.8 × 10⁻² ◯ Slightly fragile, Oligomer used Example 18 3:7◯ 6.8 × 10⁻² 5.5 × 10⁻² ◯ Oligomer used Example 19 3:7 ◯ 8.9 × 10⁻² 8.0× 10⁻² ◯ Slightly fragile, Polymer used Example 20 3:7 ◯ 5.1 × 10⁻² 4.4× 10⁻¹ ◯ Fairly flexible, Oligomer used

[0439] TABLE 1-3 Carbon-containing organic chain in theOrganic/inorganic organic/inorganic Type of Type of composite,crosslinkable composite structure Starting compound for the structurestarting acid compound (C) (A) (B) containing an acid group compoundgroup Example 21 Bis(diethoxymethylsilyl) Octamethylene3-Mercaptopropyltrimethoxysilane (E) Sulfonic octane + acidbis(ethoxydimethylsilyl) octane, Stabilized Zr complex Example 22Bis(diethoxymethylsilyl) Octamethylene 3-Mercaptopropyltrimethoxysilane(E) Sulfonic octane + acid bis(ethoxydimethylsilyl) octane, StabilizedTi complex Example 23 Bis(triethoxysilyl) Octamethylene3-Bromopropyltrimethoxysilane (H) Sulfonic octane (+ Sodium sulfite,Made protonic) acid Example 24 Bis(triethoxysilyl) Octamethylene3-Trihydroxysilylpropanesulfonic (D) Sulfonic octane acid acid Example25 Bis(triethoxysilyl) Octamethylene 3-Trihydroxysilylpropanesulfonic(D) Sulfonic octane acid acid Example 26 Bis(triethoxysilyl)Octamethylene 3-Mercaptopropyltrimethoxysilane (E) Sulfonic octane acidExample 27 Bis(triethoxysilyl) Octamethylene3-Trihydroxysilylpropanesulfonic (D) Sulfonic octane acid acid Example28 Bis(diethoxymethylsilyl) Octamethylene3-Mercaptopropyltrimethoxysilane (E) Sulfonic octane oligomer acid(C)/(8) Evaluation molar ratio Evaluation (3) in the (1) Evaluation (2)Evaluation (2) Heat starting Bending Conductivity at Conductivity atresistance at mixture test 60° C. (s/cm) 140° C. (s/cm) 140° C. RemarksExample 21 3:7 ◯ 3.3 × 10⁻² 2.8 × 10⁻² ◯ Fairly flexible, Crosslinkingagent (G) incorporated Example 22 3:7 ◯ 2.7 × 10⁻² 2.1 × 10⁻² ◯ Fairlyflexible, Crosslinking agent (G) incorporated Example 23 5:5 ◯ 1.7 ×10⁻² 1.3 × 10⁻² ◯ Flexible, Substitution adopted Example 24 5:5 ◯ 7.8 ×10⁻² 8.9 × 10⁻² ◯ White membrane, Tough Example 25 5:5 ◯ 1.8 × 10⁻² 2.5× 10⁻² ◯ Very high resistance to bending stress Example 26 3:7 ◯ 5.6 ×10⁻² 7.7 × 10⁻² ◯ High resistance to bending stress, Tough Example 275:5 ◯ 3.8 × 10⁻² 5.5 × 10⁻² ◯ Very high resistance to bending stressExample 28 3:7 ◯ 3.3 × 10⁻² 7.2 × 10⁻² ◯ High resistance to bendingstress, Semitransparent, white membrane

Comparative Example 1

[0440] An attempt was made to harden the membrane in the same manner asin EXAMPLE 1, except that 1,8-bis(tliethoxysilyl)octane was replaced by1.0 g of tetraethoxysilane. This, however, failed to produce ameasurable, self-sustaining membrane, only giving fine pieces. Theproduct could not be measured for each evaluation item. The evaluationresults and the like of the membrane are given in Table 2.

Comparative Example 2

[0441] A membrane was prepared in the same manner as in EXAMPLE 1,except that 1,8-bis(triethoxysilyl)octane was not used, but only 3.0 gof a 33% aqueous solution of 3-(trihydroxysilyl)propanesulfonic acid wasincorporated. This failed to produce a hardened membrane, because thesolution was kept fluid. The product could not be measured for eachevaluation item. The evaluation results and the like of the membrane aregiven in Table 2.

Comparative Example 3

[0442] An attempt was made to harden the membrane in the same manner asin EXAMPLE 1, except that 1,8-bis(triethoxysilyl)octane was replaced byoctyltriethoxysilane. This, however, failed to produce a measurable,self-sustaining membrane, only giving a very fragile, thin membrane. Theproduct was soluble in water, and could not be measured for eachevaluation item. The evaluation results and the like of the membrane aregiven in Table 2.

Comparative Example 4

[0443] A Nafion 117 membrane, a commercial electrolytic membrane forPEFCs, was directly used. The evaluation results and the like of themembrane are given in Table 2. TABLE 2 Carbon-containing organic chainin Organic/inorganic the composite, organic/inorganic Starting compoundfor the Type of Type of crosslinkable compound composite structurestructure (B) containing an starting acid (C) (A) acid group compoundgroup Comparative (Tetraethoxysilane) Not used 3- (D) Sulfonic Example 1Trihydroxysilylpropanesulfonic acid acid Comparative Not used Not used3- (D) Sulfonic Example 2 Trihydroxysilylpropanesulfonic acid acidComparative Octyltriethoxysilane Octamethylene 3- (D) Sulfonic Example 3Trihydroxysilylpropanesulfonic acid acid Comparative Nafion117 — — —Sulfonic Example 4 acid (C)/(8) molar Evaluation ratio in Evaluation (3)the (1) Evaluation (2) Evaluation (2) Heat starting Bending Conductivityat Conductivity at resistance at mixture test 60° C. (s/cm) 140° C.(s/cm) 140° C. Remarks Comparative 8:2 X Measurement is Measurement isMeasurement Fine pieces Example 1 impossible impossible is impossibleComparative  0:10 X Measurement is Measurement is Measurement Membranenot Example 2 impossible impossible is impossible formed Comparative 5:5X Measurement is Measurement is Measurement Soluble in Example 3impossible impossible is impossible water Comparative — ◯ 1.2 × 10⁻¹ 2.2× 10⁻² X Greatly Example 4 deformed

[0444] It is apparent, as shown in Table 1, that a proton-conductingmembrane can simultaneously achieve high proton conductivity and heatresistance, when it comprises (a) an organic/inorganic hybrid structure(A) covalently bonded to 2 or more silicon-oxygen crosslinks and, at thesame time, having a carbon atom, and (b) an acid containing structure(B) having an acid group, covalently bonded to a silicon-oxygencrosslink and having an acidic group (EXAMPLES 1 to 28). It isparticularly noted that they produce very good results in the heatresistance test (Evaluation 3) conducted at 140° C., showing essentiallyno deformation. It is also confirmed that the membrane of the presentinvention is applicable to power generation by a fuel cell (EXAMPLE 29).These performances cannot be realized unless a membrane satisfies therequirement of simultaneously containing the (a) and (b) structures asthe essential condition for the present invention. For example, it isself-evident that a membrane only containing an organic/inorganic hybridstructure (A) shows no proton conductivity in the least, although notdemonstrated by a comparative example.

[0445] It is also apparent, as shown in Table 2, that formation of amembrane in itself is difficult using an acid-containing structure (B)alone (COMPARATIVE EXAMPLE 2). Moreover, an organic/inorganic hybridstructure (A) free of carbon-containing structure as a soft componentgives a glassy membrane, which is difficult to handle and, at the sametime, cannot have a large area (COMPARATIVE EXAMPLE 1). Still more, anorganic/inorganic hybrid structure (A) free of bond which connects thecrosslinked segments to each other only gives a membrane soluble inwater, even if it has a soft component, as pointed out by Poinsignon etal. (COMPARATIVE EXAMPLE 3).

[0446] As discussed above, it is essential for the proton-conductingmembrane of the present invention to comprise both (a) anorganic/inorganic hybrid structure (A) covalently bonded to 2 or moresilicon-oxygen crosslinks and, at the same time, having a carbon atom,and (b) an acid containing structure (B) having an acid group covalentlybonded to a silicon-oxygen crosslink and having an acidic group, inorder to be durable at high temperature. A membrane can beself-sustaining to stably exhibit proton conductivity from low to hightemperature and, at the same time, to be bendable.

[0447] A fluorine-based membrane (COMPARATIVE EXAMPLE 4), which has beenused as a representative electrolytic membrane, suffers a large,irreversible deformation, although showing a high initial conductivityand relatively high conductivity after being exposed to high temperaturefor extended periods. The deformed membrane becomes hard and fragile,when dried. These observations clearly indicate that this membrane cannot be directly applied to a PEFC serviceable at high temperature.

INDUSTRIAL APPLICABILITY

[0448] The present invention provides a crosslinkable, proton-conductingmembrane having a crosslinked structure by a silicon-oxygen bond, wellserviceable at high temperature by satisfying the requirements ofcomprising

[0449] (a) an organic/inorganic hybrid structure (A) covalently bondedto 2 or more silicon-oxygen crosslinks and, at the same time, having acarbon atom, and

[0450] (b) an acid containing structure (B) having an acid group,covalently bonded to a silicon-oxygen crosslink and having an acidicgroup

[0451] The membrane of the present invention can increase operatingtemperature to 100° C. or higher for a PEFC, which has been attractingmuch attention recently, and hence can improve power generationefficiency and reduce poisoning of the catalyst by CO. Moreover, a fuelcell operating at high temperature allows to utilize its waste heat forcogeneration to produce power and heat, thus drastically enhancing itstotal energy efficiency.

What is claimed is:
 1. A proton conducting membrane crosslinkable andhaving a crosslinked structure by the silicon-oxygen bond, wherein saidproton conducting membrane comprises (a) an organic/inorganic hybridstructure (A) covalently bonded to 2 or more silicon-oxygen crosslinksand, having a carbon atom, and (b) an acid containing structure (B)having an acid group, covalently bonded to a silicon-oxygen crosslinkand having an acid group.
 2. The proton conducting membrane according toclaim 1, wherein said organic/inorganic hybrid structure (A) isrepresented by the general formula (1):

(wherein, X is an —O— bond or OH group involved in the crosslinking; R¹is a carbon-containing group of 1 to 50 carbon atoms; R² is methyl,ethyl, propyl or phenyl group; and “n” is an integer of 0, 1 or 2). 3.The proton conducting membrane according to claim 2, wherein R¹ in thegeneral formula (1) is a hydrocarbon group.
 4. The proton conductingmembrane according to claim 3, wherein R¹ in the general formula (1) hasa structure represented by the general formula (3):

(wherein, “n” is an integer of 1 to 30).
 5. The proton conductingmembrane according to claim 4, wherein said organic/inorganic hybridstructure (A) is represented by the general formula (4):

(wherein, X is an —O— bond or OH group involved in the crosslinking; and“n” is an integer of 0, 1 or 2).
 6. The proton conducting membraneaccording to claim 2, wherein R¹ in the general formula (1) has asiloxane structure.
 7. The proton conducting membrane according to claim6, wherein R¹ in the general formula-(1) is represented by the generalformula (5):

(wherein, R⁵ and R⁶ are each methyl, ethyl, propyl or phenyl, which maybe the same or different; and “n” is an integer of 1 to 20).
 8. Theproton conducting membrane according to claim 1, wherein said structure(B) containing an acid group is represented by the general formula (2):

(wherein, X is an —O— bond or OH group involved in the crosslinking; R³is a molecular chain group having at least one acid group; R⁴ is methyl,ethyl, propyl or phenyl-group;-and “m” is an integer of 0, 1 or 2). 9.The proton conducting membrane according to claim 8, wherein said acidgroup which R³ in the general formula (2) has is sulfonic acid group.10. The proton conducting membrane according to claim 9, wherein R³ inthe general formula (2) is represented by the general formula (6):

(wherein, “n” is an integer of 1 to 20).
 11. The proton conductingmembrane according to claim 10, wherein “n” in the general formula (6)is
 3. 12. The proton conducting membrane according to one of claims 1 to11 which is further composited with a fibrous material (I).
 13. Theproton conducting membrane according to claim 12, wherein said fibrousmaterial (I) is composed of a short fibrous material (J) and/or longfibrous material (K).
 14. The proton conducting membrane according toclaim 12, wherein said fibrous material (I) is surface-treated with asilane coupling agent to have a proton-conductive surface.
 15. Theproton conducting membrane according to claim 12, wherein said fibrousmaterial (I) is composed of glass fibers.
 16. The proton conductingmembrane according to claim 15, wherein said glass fibers are of alkali-or acid-resistant glass.
 17. The proton conducting membrane according toclaim 13, wherein said long fibrous material (K) is composed of glassfibers in the form of woven fabric, non-woven fabric or glass fiberpaper produced by a paper-making process.
 18. The proton conductingmembrane according to claim 17, wherein said long fibrous material (K)is in the form of thirled, square-weave fabric.
 19. The protonconducting membrane according to claim 17, wherein said long fibrousmaterial (K) has a thickness of 300 μm or less.
 20. The protonconducting membrane according to claim 12, wherein said short fibrousmaterial (J) is incorporated at 1 to 75% by weight on theorganic/inorganic hybrid structure (A) and acid-containing structure (B)totaled.
 21. The proton conducting membrane according to claim 13,wherein said short fibrous material (J) is composed of whiskers (L)and/or short glass fibers (M).
 22. The proton conducting membraneaccording to claim 21, wherein said whiskers (L) have a diameter of 0.1to 3 μm, length of 1 to 20 μm and aspect ratio of 5 to
 100. 23. Theproton conducting membrane according to claim 21 or 22, wherein saidwhiskers (L) are of boron carbide, silicon carbide, alumina, aluminumborate, silicon nitride or K₂O.6TiO₂.
 24. A method for producing theproton conducting membrane of one of claims 1 to 23, comprising steps ofpreparing a mixture containing an organic/inorganic hybrid,crosslinkable compound (C) and compound (D), the former having 2 or morecrosslinkable silyl groups and carbon atoms each being bonded to thesilyl group via the covalent bond and the latter having a crosslinkablesilyl group and acid group, as the first step; forming the above mixtureinto a film as the second step; and hydrolyzing/condensing or onlycondensing the hydrolyzable silyl group contained in the mixture formedinto the film to form a crosslinked structure as the third step.
 25. Amethod for producing the proton conducting membrane of one of claims 1to 23, comprising steps of preparing a mixture containing anorganic/inorganic hybrid, crosslinkable compound (C) and compound (E),the former having 2 or more crosslinkable silyl groups and carbon atomseach being bonded to the silyl group via the covalent bond and thelatter having a crosslinkable silyl group and mercapto group, as thefirst step; forming the above mixture into a film as the second step;hydrolyzing and condensing the hydrolyzable silyl group contained in themixture formed into the film to form a crosslinked structure as thethird step; and oxidation of the mercapto group in the crosslinkedstructure obtained in the third step into sulfonfic acid as the fourthstep.
 26. A method for producing the proton conducting membrane of oneof claims 1 to 23, comprising steps of preparing a mixture containing anorganic/inorganic hybrid, crosslinkable compound (C) and compound (F),the former having 2 or more crosslinkable silyl groups and carbon atomseach being bonded to the silyl group via the covalent bond and thelatter having a crosslinkable silyl group and polysulfide group, as thefirst step; forming the above mixture into a film as the second step;hydrolyzing and condensing the hydrolyzable silyl group contained in themixture formed into the film to form a crosslinked structure as thethird step; and oxidation of the polysulfide group in the crosslinkedstructure obtained in the third step into sulfonic acid as the fourthstep.
 27. A method for producing the proton conducting membrane of oneof claims 1 to 23, comprising steps of preparing a mixture containing anorganic/inorganic hybrid, crosslinkable compound (C) and compound (H),the former having 2 or more crosslinkable silyl groups and carbon atomseach being bonded to the silyl group via the covalent bond and thelatter having a crosslinkable silyl group and halogen group, as thefirst step; forming the above mixture into a film as the second step;hydrolyzing and condensing the hydrolyzable silyl group contained in themixture formed into the film to form a crosslinked structure as thethird step; and substitution of the halogen group in the crosslinkedstructure obtained in the third step with sulfonic acid group as thefourth step.
 28. The method according to one of claims 24 to 27 forproducing the proton conducting membrane, wherein said organic/inorganichybrid, crosslinkable compound (C) is represented by the general formula(7):

(wherein, R¹ is a carbon-containing group of 1 to 50 carbon atoms; R² ismethyl, ethyl, propyl or phenyl group; R⁵ is Cl, or OCH₃, OC₂H₅, OC₆H₅,OH or OCOCH₃ group; and “n” is 0, 1 or 2).
 29. The method according toclaim 28 for producing the proton conducting membrane, wherein R¹ in thegeneral formula (7) is a hydrocarbon group.
 30. The method according toclaim 29 for producing the proton conducting membrane, wherein saidorganic/inorganic hybrid, crosslinkable compound (C) is represented bythe general formula (8):

(wherein, R² is methyl, ethyl, propyl or phenyl group; R⁵ is Cl, orOCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is an integer of 1 to 30;and “n” is 0, 1 or 2).
 31. The method according to claim 30 forproducing the proton conducting membrane, wherein said organic/inorganichybrid, crosslinkable compound (C) is represented by the general formula(9):

(wherein, R⁵ is OCH₃ or OC₂H₅ group; and “n” is 0, 1 or 2).
 32. Themethod according to claim 28 for producing the proton conductingmembrane, wherein said organic/inorganic hybrid, crosslinkable compound(C) is represented by the general formula (10):

(wherein, R¹¹, R¹², R¹³ and R¹⁴ are each methyl, ethyl, propyl or phenylgroup, which may be the same or different; R⁵ is Cl, or OCH₃, OC₂H₅,OC₆H₅, OH or OCOCH₃ group; “m” is an integer of 1 to 30; and “n” is 0, 1or 2).
 33. The method according to claim 32 for producing the protonconducting membrane, wherein said organic/inorganic hybrid,crosslinkable compound (C) is represented by the general formula (11):

(wherein, R⁵ is Cl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is aninteger of 1 to 30; and “n” is 0, 1 or 2).
 34. The method according toclaim 24 for producing the proton conducting membrane, wherein said acidcontaining compound (D) is represented by the general formula (12):

(wherein, R³ is a molecular chain group having at least one acid group,R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, or OCH₃, OC₂H₅,OC₆H₅, OH or OCOCH₃ group; “m” is 0, 1 or 2).
 35. The method accordingto claim 34 for producing the proton conducting membrane, wherein saidacid containing compound (D) is sulfonic acid group.
 36. The methodaccording to claim 35 for producing the proton conducting membrane,wherein said acid containing compound (D) is represented by the generalformula (13):

(wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, orOCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; X is hydrogen, an alkali metal,alkyl group or ammonium group; “m” is 0, 1 or 2; and “n” is an integerof 1 to 20).
 37. The method according to claim 36 for producing theproton conducting membrane, wherein “n” in the general formula (13) is3.
 38. The method according to claim 24 for producing the protonconducting membrane, wherein said organic/inorganic hybrid,crosslinkable compound (C) and acid containing compound (D) areincorporated in a mixing ratio of 9:1 to 1:9 by weight.
 39. The methodaccording to claim 25 for producing the proton conducting membrane,wherein said compound (E) having mercapto group is represented by thegeneral formula (14):

(wherein, R⁷ is a molecular chain group having at least one mercaptogroup; R⁴ is methyl, ethyl, propyl or phenyl group; Rr is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).
 40. The methodaccording to claim 39 for producing the proton conducting membrane,wherein said compound (E) having mercapto group is represented by thegeneral formula (15):

(wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, orOCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m” is 0, 1 or 2; and “n” is aninteger of 1 to 20).
 41. The method according to claim 25 for producingthe proton conducting membrane, wherein said compound (E) havingmercapto group is represented by the general formula (16):

(wherein, R⁶ is H, or CH₃, C₂H₅, C₃H₇ or C₆H₅ group; R⁴ is CH₃, C₂H₅,C₃H₇, C₆H₅, OH, OCH₃, OC₂H₅, OC₆H₅ group, or O—Si bond; “m” is aninteger of 1 to 20; and “n” is an integer of 3 to 500).
 42. The methodaccording to claim 41 for producing the proton conducting membrane,wherein R⁴, “m” and “n” in the general formula (16) are OCH₃ group, 3and an integer of 3 to 100, respectively.
 43. The method according toclaim 41 for producing the proton conducting membrane, wherein R⁴, “m”and “n” in the general formula (16) are CH₃ group, 3 and an integer of 3to 300, respectively.
 44. The method according to claim 25 for producingthe proton conducting membrane, wherein said compound (E) havingmercapto group is represented by the general formula (17):

(wherein, R⁶ is H, or CH₃, C₂H₅, C₃H₇ or C₆H₅ group; R⁴ is CH₃, C₂H₅,C₃H₇, C₆H₅, OH, OCH₃, OC₂H₅ or OC₆H₅ group, R¹¹ is a substitute of 6carbon atoms or less; “m” is an integer of 1 to 20; “n” is an integer of3 to 500; and “n+x” is an integer of 500 or less, where the unitcontaining mercapto group and that containing R¹¹ may be present in ablock or random form).
 45. The method according to claim 44 forproducing the proton conducting membrane, wherein R⁴, “m” and “n+x” inthe general formula (17) are OCH₃ group, 3 and an integer of 50 or less,respectively.
 46. The method according to claim 25 for producing theproton conducting membrane, wherein said organic/inorganic hybrid,crosslinkable compound (C) and compound (E) having mercapto group areincorporated in a mixing ratio of 9:1 to 1:9 by weight.
 47. The methodaccording to claim 26 for producing the proton conducting membrane,wherein said compound (F) having a polysulfide group is represented bythe general formula (18):

(wherein, R⁸ is a molecular chain group having at least one polysulfidegroup; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).
 48. The methodaccording to claim 47 for producing the proton conducting membrane,wherein said compound (F) having a polysulfide group is represented bythe general formula (19):

(wherein, R⁹ is a molecular chain group having at least one polysulfidegroup; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).
 49. The methodaccording to claim 48 for producing the proton conducting membrane,wherein said compound (F) having a polysulfide group is represented bythe general formula (20):

(wherein, R¹⁰ is a polysulfide group; R⁴ is methyl, ethyl, propyl orphenyl group; R⁶ is Cl, or OCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; “m”is 0, 1 or 2; and “n” is an integer of 1 to 6).
 50. The method accordingto claim 49 for producing the proton conducting membrane, wherein “n” inthe general formula (20) is
 3. 51. The method according to one of claims47 to 50 for producing the proton conducting membrane, wherein saidpolysulfide group is tetrasulfide group (—S—S—S—S—).
 52. The methodaccording to one of claims 47 to 50 for producing the proton conductingmembrane, wherein said polysulfide group is disulfide group (—S—S—). 53.The method according to claim 26 for producing the proton conductingmembrane, wherein said organic/inorganic hybrid, crosslinkable compound(C) and compound (F) having a polysulfide group are incorporated in amixing ratio of 95:5 to 10:90 by weight.
 54. The method according to oneof claims 24 to 27 for producing the proton conducting membrane, whereina crosslinking agent (G) of hydrolyzable, metallic compound is used forthe first step.
 55. The method according to claim 54 for producing theproton conducting membrane, wherein said crosslinking agent (G) is of acompound represented by the general formula (21):

(wherein, R⁶ is CH₃ or C₂H₅ group; and “m” is an integer of 1 to 300).56. The method according to claim 54 for producing the proton conductingmembrane, wherein said crosslinking agent (G) is of a hydrolyzable,metallic compound having Ti, Zr or Al.
 57. The method according to claim27 for producing the proton conducting membrane, wherein said compound(H) having a halogen group is represented by the general formula (22):

(wherein, R¹² is a molecular chain group having at least one halogengroup; R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, or OCH₃,OC₂H₅, OC₆H₅, OH or OCOCH₃ group; and “m” is 0, 1 or 2).
 58. The methodaccording to claim 57 for producing the proton conducting membrane,wherein said compound (H) having a halogen group is represented by thegeneral formula (23):

(wherein, R⁴ is methyl, ethyl, propyl or phenyl group; R⁶ is Cl, orOCH₃, OC₂H₅, OC₆H₅, OH or OCOCH₃ group; X is Cl, Br or I; “n” is aninteger of 1 to 20; and “m” is 0, 1 or 2).
 59. The method according toclaim 27 for producing the proton conducting membrane, wherein saidorganic/inorganic hybrid, crosslinkable compound (C) and compound (H)having a halogen group are incorporated in a mixing ratio of 9:1 to 1:9by weight.
 60. The method according to one of claims 24 to 27 forproducing the proton conducting membrane, wherein a step for aging at100 to 300° C. is included as a post-treatment step.
 61. The methodaccording to one of claims 24 to 27 for producing the proton conductingmembrane, wherein said short fibrous material (J) is incorporated in themixture in the first step, when it is incorporated as said fibrousmaterial (I) to be composited with the proton-conducting membrane. 62.The method according to one of claims 49 to 60 for producing the protonconducting membrane, wherein said long fibrous material (K) is loaded inthe second step with the mixture obtained in the first step, when it isincorporated in the form of sheet as said fibrous material (I) to becomposited with the proton-conducting membrane.
 63. A fuel cell whichuses the proton conducting membrane according to one of claims 1 to 23.